This is an archive of past discussions about Light. Do not edit the contents of this page. If you wish to start a new discussion or revive an old one, please do so on the current talk page.
gogi "Light has a mass. The foton has a mass, i know this because the light slow down when moving trought sea, and large object like eart bend the light when passing near bay. Bending of the light does not have nothing with bending of the space."
tatiana bradley
As far as I understand it, the evanescent wave coupling stuff is the same as the other group velocity > c stuff in the case of extreme absorption.
There seems to be a lot of interest and half-explainations of this topic scattered around the wikipedia, I may try to write a superluminal communication article, In My Copious Free Time.
What's really interesting is the effect that occurs between the plates of a Casimir Force experiment, that's the only legitimate v>c stuff I know about. -- DrBob
How is "velocity" incorrect? It was good enough for Einstein:
In short, let us assume that the simple law of the constancy of the velocity of light c (in vacuum) is justifiably believed by the child at school. [1]
I suppose we should make it clear that we are referring to the velocity of the propagation of light. --TheCunctator
Well, if the velocity of light was a constant, then it would always move in the same direction. (Velocity being a vector quantity.) This is not true for all observers (the direction of light propogation being different in different reference frames, e.g. the light-clock thought experiments), and not even true for a single observer (light being able to go in any direction). However, the speed of light, being the magnitute of the velocity, is the same for all observers.
Maybe I'm being nit-picky but it seems that Einstein was being loose with the terminology in the article you reference. I still think 'speed' is the most technically correct. -- DrBob
I agree, velocity is generally taken to be a vector quantity and speed is a scalar. The speed of light in vacuum is constanct and equal to c; the velocity vector of light in vacuum is not constant, because light can travel in different directions; the magnitude of the velocity vector is c. --AxelBoldt
Fine with me. I think it would be fair to note that Einstein used "velocity", and define speed. --TheCunctator
Are light-years really the prefered unit in Astronomy? I have heard that they are mostly used in popular science articles, but real astronomers use Parsecs instead. I'm not a real astronomer, so i can't vouch for this. -- Geronimo Jones.
The star and galaxy catalogs tend to use parsecs rather than light-years, so I'd guess it's a safe bet.
Yep, astronomer use parsecs (just over three light years) for light year sized measurements, with MegaParsecs, KiloParsecs etc also being used. This is due to parsecs coming from the parallactic measurements techinique of determining distances from angles of parallax - ParSec = one Prallatic Second of arc, where a second here is one sixtieth of a minute, which is one sixtieth of a degree. For larger scales, doppler shifts (z) are used.--MilleauRekiir22:53, 12 May 2006 (UTC)
"Light passes through liquids such as water or solids such as glass at reduced volocity."
Good point, but I have three reservations:
1 - It's "velocity", not "volocity"
2 - I think it should be "speed", not velocity. Velocity is a vector, right? (Somebody help me out here :-) )
3 - Maybe this could go in the paragraph here about the speed of light, or in the entry on speed of light linked there
- Thanks :-)
All good points, I had a lot of trouble that day getting on to Wikipedia. If I can keep getting in I'll try to do something better. My question is why you took all this time on talk and didn't just do a better job including the material.
V for velocity?
In the section on speed of light, the symbol v - for velocity? :) - renders in my browser as a greek letter nu, which could cause confusion as that is commonly used to represent frequency. This happens throughout except in the last equation, v=c/n , where it is rendered correctly as v, but the whole equation is in larger type. I couldn't find anything to explain this in the markup, except that the last equation had spaces around the "=" whereas the first two did not.
Irrelevant as this seemed, I put spaces in the first two and, in preview, the problem was fixed - v rendered as v, and all equations similar (large) font. However, when I saved the changes, it reverted to the earlier rendering, apart from the added spaces.
Anyone have an idea what's going on and how to fix it? Do others see the same problem or does it display normally for you? (I'm using Opera 7.23, Windows). I've also had a look in Internet Explorer and seen similar, except a slight difference in font appearance makes me wonder whether I'm seeing, not a Greek "nu" but a cursive "v". Either way, it is confusing.
--Richard Jones 16:16, 13 Mar 2004 (UTC)
I think it is not a nu but a v in italics; that is what TeX rendered as PNG gives, as opposed to TeX rendered as HTML. For more uniformity specify HTML if possible in the preferences. --Patrick 20:56, 13 Mar 2004 (UTC)
The discussion of infra-red (IR) and infra-red cameras is incomplete. Near infra-red is just outside the range of humans vision (730nm - 1100nm), and most infra-red (night-vision) cameras detect these wavelengths. Actually all CCD - CMOS cameras will detect these wavelengths, though normally a filter is put on top of the image sensor to stop wavelengths greater than 700nm from reaching the cameras sensor. Infra-red (forget the exact wavelength, believe between 2000nm - 5000nm?) is detected as heat. It requires a completely different kind of camera / sensor to detect these wavelengths, and these cameras are quite expensive.
Explanation on using Jupiter's moons to calculate speed of light confuses me
I was confused by this explanation:
when Earth and Jupiter were not as close, the moon's revolution seemed to be more. It was clear that light took longer to reach Earth when it was farther away from Jupiter. The speed of light was calculated by analyzing the distance between the two planets at various times.
Why would the moon's revolution seem to be more? Let's take a revolution when Jupitar is near Earth starting at n1 and ending at n2. Then, consider a revolution and when Jupiter is far from Earth starting at f1 and ending at f2. Ok, so it takes at extra delay d for the light of the start of the revolution to reach Earth when f1 so, in fact, the revolution started at time f1-d. But so what? The revolution ends at nearly time f2-d also. Thus:
(f2-d) - (f1-d) = f2 - f1 = period of revolution = n2 - n1
Distance from Earth has no bearing.
After thinking for a moment, I have a guess at what the explation was trying to say.
When Jupiter and Earth are moving apart, the revolutions of the moon would seem to take more time. Similarly, the revolutions would seem to speed up as Jupiter and Earth become closer. That is, a distant Jupiter moving closer would seem to have faster moons whereas a nearby Jupiter moving away would seem to have slower revolutions.
That is, relative motion and notdistance is what changes the apparent period of the moon's revolution. If I am correct, then a better explanation would be:
when Earth and Jupiter were moving apart from each other, the moon's revolution seemed to take longer. It was clear that light took longer to reach Earth because Earth and Jupiter had moved apart during the start and the end of the moon's revolution. Similarly, as Earth and Jupiter came closer together, the moon's revolution seemed to take less time. The speed of light was calculated by analyzing the distance between the two planets at various times.
However, maybe I'm being a bit too wordy. How about:
when Earth and Jupiter were moving apart from each other, the moon's revolution seemed to take longer. It was clear that light took longer to reach Earth as Earth and Jupiter moved apart. Similarly, as Earth and Jupiter came closer together, the moon's revolution seemed to take less time. The speed of light was calculated by analyzing the distance between the two planets at various times.
So, would some astronomer confirm/reject my suggestion?
Anonymous -Sorry I'm no astronomer but these test are invalid. Around the time of Einstien scientist conducted a test to find the speed of light. the used this equation s-(c-s)=c
s= earth speed approaching the sun
c= the speed of light
6 months later when the Earth was moving away they did the tests over again and got the same results. Then Einstein developed the theory of special relativity. I'd suggest you look at that article.
I've restructured the article somewhat, to move the Theories of light down to the bottom, as the todo and WP:FAC comments suggest, plus other general tidying, also deleting one of the prism images. Some parts are clearly in need of more work. -- ALoan(Talk) 14:43, 11 Nov 2004 (UTC)
Should we place the headlines in the article from the Todo-list above? That way it would be easier to edit the article into new subsections. Thechamelon 12:52, Apr 19, 2005 (UTC)
lightyear as an unit of measure
light-year is a deprecated but accepted unit. The only unit that should be used in SI should be the meter. The unit parsec is strongly preferred to light-year. Since light-year unit is, to some degree, common used, it worth to explain what it is.
Quantum Electrodynamics
I think a brief explanation of this theory--or at least a link to the related Wikipedia entry--would be appropriate, given that it has resolved the wave-particle duality of light, which this article incorrectly describes as the modern theory of light.
Does fluorescence go here?
Hey, in the section about ways to quantify light or measure light: plese don't forget the Spectral power distribution. It is a graph/plot generated by a spectroradiometer reading the Watt/Flux...something at 31 regions along the spectrum from 400 to 700nm.--done
I can help. I need the definition bad.--[[User:Dkroll2|Dkroll2]] 22:13, Dec 14, 2004 (UTC)
Scientific notation or Engineering notation?
The article currently contains the following statement:
Light is the visible portion of the electromagnetic spectrum, between the frequencies of 3.8×1014 hertz (abbreviated 'Hz') and 7.5×1014 Hz. Since the speed (v), frequency (f or ?), and wavelength (?) of a wave obey the relation...
I don't know about you, but I always find I have a much faster "intuitive" grasp of a quantity if it's stated in "Engineering Notation" (such as "380 THz" or maybe "380×1012 Hz") rather than pure Scientific Notation ("3.8×1014 Hz"). I think it's because I routinely work with kilo, mega, giga, tera, and with nanometers and the rest of the SI units and prefixes but not with pure scientific notation. Does anyone else agree with this? And do you agree with it to the extent that we change the article to use this form?
I don't think there can be any objections to using SI-standard abbreviations. Not so sure about Engineering notation in the sense of using the raw unit but constraining the exponent of ten to be a multiple of a 3. It's offered e.g. on HP calculators, but is it really used in engineering publications? Dpbsmith(talk) 15:11, 23 Apr 2005 (UTC)
I've only seen use of prefixes. - Omegatron 17:37, Apr 23, 2005 (UTC)
I guess I'm arguing in favor of using the SI prefixes rather than ANY scientific or engineering notation.
How about: "Light is the visible portion of the electromagnetic spectrum, between the frequencies of 380 THz (3.8×1014hertz) and 750 THz (7.5×1014hertz)." The THz link goes to the SI multiples of Hertz, so one could even omit the info in parenthesis. Splarka 01:09, 24 Apr 2005 (UTC)
Snip Snip. (Since we all seem to be in agreement) Splarka 19:28, 24 Apr 2005 (UTC)
A question about light
Someone had already posted "Visible Light: No Such Thing" above, and it turned out to be a bit different than what I want to ask:
Is light actually visible? I'm looking at my desk lamp now. The light bulb is illuminated and casts light onto the desk and the various items thereon. But I don't actually see the light emitting from the bulb: I see the bulb itself, and the items that are lit. This causes me to guess that we don't actually see light, we see objects that either emit or receive light. (The only exception I can think of would be a laser). Maybe someone here can tell me if I'm way off. -- Gyrofrog (talk)07:49, 24 August 2005 (UTC)
You "see" an object when your brain interprets the signals coming from your eyes. The signals come from the rods and cones in your eyes when they detect photons. The photons come from objects that emit light, either direct from the light source or after the photons are bounced off or pass through an intermediate object or medium. -- ALoan(Talk)10:59, 24 August 2005 (UTC)
I'm not sure this is a very meaningful question. However, it may be worth noting that light, being a vector field, always has directionality associated with it. For example, you do not see a "beam" of light directly when viewed from the side. If a laser beam is shining in a vacuum, you cannot see it at all from the side, no matter how bright it is. When a light source emits a beam of light, you can see it directly if it is shining straight at you, but it does not look alike a beam, it looks like a very bright starlike point. From the side, you only see a "beam" if there is something in the beam that can scatter or reflect it out of the beam. Perhaps this is what you are thinking of.
Decades ago, when it was usual for people to smoke in movie theatres, you could look up and see a very solid-looking cone of light, with moving cones of light, dark, and color within it. Now the projector beam is almost invisible. Dpbsmith(talk)15:28, 24 August 2005 (UTC)
This was on Qi, Stephen Fry said that light is NOT visible only its effects are visible. The easiest way to think about it is if you could see light, that's all you'd see. Think about it......
roomfilling density of lightbeams from everywhere ?
What keeps on striking me is that lightbeams (in a lighted room or in the landscape) come reflected from everywhere to everywhere. At any point lightbeams must be crossing one another from every direction (except of course inside objects). It's like connecting every point on a sheet with every other with a pencil .. just a big mess ..
In just one point all reflected beams meet, and alike in every other point ..
How is it possible, that all those beams don't hinder / interact / mess up / interfere (?) one another when thinking of them as electromagnetic wave 'packages' (packs, packets, light quantum )? Is it because they are 'constituted' - as far as actual theory is concerned - .. constituted only of the abstract notion of their 'fields' (electromagnetic field) propagating through space? If so, then .. What 'is' a lightbeam? Or, what is it thought of by theory?
Is it because they all have a slightly different direction of propagation?
(The corresponding article: electromagnetic interaction is very meager when read on this special behalf of free travelling electromagnetic waves (i'd rather cling to everyday's "lightbeams") crossing one another, and other quantum electrodynamics -articles mostly deal with charged particles, rather than em-waves)
Or do I just not understand , what a field is or how it can constitute a real lightbeam?
You're exactly right, and that is a good question, in fact a superb question. I'm going to answer your question with another question. What does a single ray of light look like? It does not look like a sci-fi movie of a laser beam. That's not the beam itself. When you "see" a beam of light from the side, and it looks like a bright line, what you are really seeing is millions of other beams of light resulting from the beam itself being scattered by air molecules or particles of dust or smoke. Dpbsmith(talk)10:41, 28 September 2006 (UTC)
I think the big reason you don't see all sorts of interesting interference effects is that most light that fills most rooms is completely non-coherent (uncorrelated in phase, frequency, or polarization), so any interference effects that occur happen at a microscopic scale where you never notice them.
I'd imagine that it would be pretty interesting to experiment with a room full of well-correlated coherent light. A small-scale example of this occurs when you defocus the beam from a laser pointer so that it creates a large-ish spot on the wall. You will see all sorts of interesting "grain" effects in that spot; these are a result of constructive and destructive interference. (I don't know if it's true, but I've been told that near-sighted (mypoic) people see the grain pattern move one way when they move their heads and far-sighted (hyperopic) people see the grain structure move the other way, but I can never remember which type of vision gets which type of motion.)
Unless you're at extremely high intensities (when effects from quantum electrodynamics become important), light beams in a vacuum don't interact except by a linear superposition of their electric fields. So at the point they meet you can get interference effects (if the beams are sufficiently coherent), but there's no lasting effect on the beam and one will pass straight through the other as if it wasn't there. As Atlant says, for ordinary light sources the effects of interference aren't very noticable. If you illuminate a large area with a laser, you tend to see speckle patterns, which is simply due to inteference between different parts of the beam.
Ok, incoherence, no linear superposition, interference only in the meeting point, but passing it unchanged .. that explains it.
Light is a wonderous phenomenon, trying to understand the nature of the universe.
[seeing unclear under water stuff deleted by roeighty]
roeighty
13:50, 5 October 2005 (UTC)
I'd say that the concept of "light beams" should be used to help students learn about angles of refraction and reflection, and should then be discarded. If you shine some light from a torch/flashlight in a dusty room, you see a "beam" reflected in the dust particles. This beam is a cone-shaped volume of space that is filled with a series of spherical wavefronts radiating from the source. The wavefronts are the aggregates of the amplitudes of the wave functions of countless photons. You can make this cone as narrow as you like, but it never turns into a pure linear "beam"; it's always a cone. Even laser beams are just very narrow cones. It's only classical optics experiments that fool us into thinking that light consists of "beams". Thinking in terms of "beams" makes it harder to understand what's going on in the world of photons. --Heron20:53, 22 July 2006 (UTC)
I disagree. Beam is the layman way of says k-vector, which is very fundamental in the world of photons. If your beam is really a cone, it has several k-vectors and therefore is not a 'beam'. What you are saying is similar to saying we shouldn't use the word frequency, because most light is actually has some finite spectral bandwidth and therefore using that word makes understanding 'the world of photons' difficult. Waxigloo18:13, 27 September 2006 (UTC)
A light wave?That's an interesting explanation. I didn't know about k-vectors, so I didn't know that beams had physical reality. Are you saying, then, that a photon occupies only one spatial dimension? And is a picture of a 'light wave', like the one opposite, actually a picture of a photon, or part of a photon as it whizzes through the picture from left to right, or of the sum of lots of photons, or something else? --Heron20:12, 27 September 2006 (UTC)
I think bringing the word photon into the picture just makes the discussion more complex that it needs to be. There are very few cases where quantization of the field need be applied, and a discussion of k-vectors is not one of them.
The k-vector is basically the momentum of the light and usually points in the direction of propagation. So, you run into an uncertainty principle (x and p are conjugate related by Fourier transform) kind of issue here: if the k-vector is very well defined, then the position x is not. For example: a plane wave has a single k-vector, but occupies all space (a very poorly defined x -- hence the name: its wavefronts are infinite parallel planes).
My previous explanation was a bit simplistic, and to some extent you are right: there is no light occupying a single x and a single p and thus acting like a perfect beam. But geometric optics (based on beams) is very handy, because all the ray matrices developed there can be applied to Gaussian beams, a much more realistic description of beam propagation (under the paraxial approximation).Waxigloo22:06, 27 September 2006 (UTC)
Thanks again. I'm happy with the classical explanation, now I know that it does not claim the existence of perfectly one-dimensional beams. --Heron19:53, 28 September 2006 (UTC)
Beams of light follow "straight lines" ("light-like" [geodesic]]s) in space-time. Massive objects effectively cause space-time to be bent, so the "straight lines" are themselves curves. For example, where a massive object, such as a cluster of galaxies falls along the sightline from Earth to a distance astronomical object, such as a quasar, the massive object can bend the light like a lens, a phenomenon known as gravitational lensing. HTH. -- ALoan(Talk)12:30, 22 November 2005 (UTC)
There must be equations for the bending of light that don't use curved geometry. Can the bending of light by gravity be directly related to its energy. For example, if I was going to make an educated guess of how light bends, I would start with the equation accelleration = m G/r^2 and transform it into A = E G/(rc)^2. Then using that I could predict how light would be bent. Using relativity, the speed wouldn't change, but the direction would. It would also predict that the bending of light depends on its frequency (which is proportional to its energy). I have no idea if this is even near right, but the equations for how light is affected by gravity would be very useful to have somewhere around here. Fresheneesz19:17, 12 April 2006 (UTC)
I was taught in imaging science to never use these 2 words together ; Visible Light
It is redundant.
If it is Light then it is by definition "Visible"
If it is visible then there is some light going on in the equation.
Visible portion or region of the electromagnetic spectrum is ok.
Or just Light. Light Source, etc.
I know it seems short and lonely, but it is correct.
I think it is too common to use "light" when referring to any type of EM radiation used for the purpose of illumination, not just directly visibly. For example, we say "infrared light" often, "infrared radiation" is kind of cumbersome when referring to the frequencies near to visible. If an invisible source of IR radiation is used in conjunction with night vision goggles, it becomes a light source for the camera. Some insects can see "UV light". "UV Radiation" is used when describing ionizing UV that causes cancer, sun tans, and sterilizing of bacteria. "UV Light" is used when referring to near-visible illumination (as in fluorescence, or when describing the visible spectrum of some insects). If you want to argue semantics into the ground then "Visible Light" is incorrect. But if you want to get by outside a physics class, just accept that the phrase "Visible Light" == "Light visible to native human eyes", and that other forms of light exist. 64.162.10.163 21:52, 28 Mar 2005 (UTC)
The word "light" is not limited only to wavelengths visible to human beings. Technically, light is electromagnetic radiation of optical frequencies, which include not only visible wavelengths, but also ultraviolet and infrared. It is perfectly natural to speak of "ultraviolet light" or of "infrared light." Moreover, does the term "sunlight" really refer only to the wavelengths that humans can see? As we all know, that is simply not true. Sunlight includes electromagnetic radiation from the entire spectrum, with the vast majority of the energy density coming from infrared, visible, and ultraviolet light. From a scientific point of view, it is rather parochial of us humans to say that light includes only wavelengths that we can see, as if the others don't really exist. As it turns out, not all animals are limited to the same region of the spectrum that we are. Honeybees, for example, can see ultraviolet light, and many flowers exhibit different "color" patterns in UV from what we see in the visible region. First Harmonic12:07, 7 September 2006 (UTC)
Technically speaking, the entire electromagnetic spectrum consists of three major regions: (1) radio, (2) optical, and (3) high energy. Radio includes long waves, short waves, and microwaves. Optical includes infrared light, visible light, and ultraviolet light. And high energy includes X-rays and gamma-rays. Although establishing the exact boundaries between the regions is somewhat arbitrary (the regions overlap to some degree), the distincition between the regions is not arbitrary. There are real and fundamental differences in the physical properties between these three regions. By comparison, the only real difference between red light of wavelength 650 nm and infrared light of wavelength 850 nm is the wavelength itself, and the fact that human beings can detect one but not the other. First Harmonic12:07, 7 September 2006 (UTC)
Visible Light is "oily oil"
I absolutely agree with the first definition:
"LIGHT is electromagnetic radiation with a wavelength that is VISIBLE to the EYE".
But NOBODY from scientists use the word "Light" as "electromagnetic radiation of any wavelength".
UV radiation is NOT light! IR radiation is NOT light! Radiowaves is NOT light! X-ray is NOT light! But all of them are electromagnetic radiation with different wavelengths.
But NOBODY from scientists use the word "Light" as "electromagnetic radiation of any wavelength".
UV radiation is NOT light! ... X-ray is NOT light!
You're going to have an awfully hard time explaining the naming of the National Synchrotron Light Source then, as it emits infrared, vacuum UV light, and soft X-rays, none of which are called "light" by real scientists according to you. I guess the guys at Brookhaven National Laboratory aren't real scientists, ehh? ;-)
In my experience scientists and engineers frequently do talk about "ultraviolet light" or "infrared light"; but on the other hand I don't know anyone who uses the term light to encompass all electromagnetic radiation. I would say in a technical context, light is the subject of the study of optics. It's em radiation at wavelengths that historically were studied by optical methods.
The Photon is absolutely correct. The word light referes to that portion of the electromagnetic spectrum that is the subject of the science of optics. The study of optics generally encompasses not only visible light, but also infrared light and ultraviolet light. In my experience, very few scientists or engineers would be confused by the term "infrared light" or "ultraviolet light", nor would they find anything objectionable in those terms. The fact is that the electromagnetic spectrum consists of three main regions: radio waves at the low frequency end of the spectrum, high energy radiation at the high frequency end of the spectrum, and light or optical frequencies in the middle. First Harmonic (talk) 18:05, 29 June 2008 (UTC)
Well, there's always the dictionary:
"1. Physics a. Electromagnetic radiation that has a wavelength in the range from about 4,000 (violet) to about 7,700 (red) angstroms and may be perceived by the normal unaided human eye. b. Electromagnetic radiation of any wavelength." [2]
Just to shed some invisible light on the subject, the near infrared and near UV certainly are visible. The sensitivity of the eye tails off gradually and slowly without a sharp cutoff. Probably one reason for limiting it to the conventional 400-700 nm is that the sensitivity outside this range is very variable from person to person. Of course, astronauts perceive cosmic rays as flashes of light when they hit the retina,[3] so in a sense cosmic rays are visible, too. Dpbsmith(talk)15:50, 7 September 2006 (UTC)
You reminded me of a story from a professor of mine. While he was in graduate school they had a beam off an accelerator. They used to check that their beam was on by sticking an eyeball down in its path and looking for flashes of light from bremsstrahlung. — Laura Scudder☎16:41, 7 September 2006 (UTC)
Weight of light
because light has infinite mass does it have a weight of zero?
Weight is a force. "Weight" is usually used to mean the force which an object feels as a result of gravity. I've never heard anyone refer to the weight of light, but light does NOT have infinite mass - just the opposite. Light has zero rest mass - a slightly misleading term for light, since light is never at rest. I'm a little fuzzy on the "mass of light" issue myself, but "weight" is never ever discussed in the context of light or particle physics, because it simply isn't used as a technical term. Fresheneesz19:27, 12 April 2006 (UTC)
I believe that the increase in mass with speed is a multiplicative process, and not an additive one; thus, if a particle has a rest mass of zero, it will have no mass per se, no matter what its speed is. However, I am only an amateur physicist (I'm a high school freshman), so I would not take this as definitive. Geekman314(contact me)02:20, 18 February 2007 (UTC)
The rest mass is zero, and the multiplier at speed c is infinity, so the mass is indeterminate from that approach. You need to use E=mc2 instead, to get the mass from the energy of the photon, or from the total light energy. That mass is attracted by gravity like ordinary mass, so you can compute the weight of light. Dicklyon05:14, 18 February 2007 (UTC)
Since the gravitational field of stars deflects light, my guess is that light does have weight, but I'm not speaking from much knowledge. Dpbsmith(talk)15:53, 7 September 2006 (UTC)
Physical Review Focus has an article called The Weight of Light which references articles in a journal about the history of physics entitled Weighing Photons, so apparently the definition of "weight" used by physicists is consistent with the idea that light has weight. Dpbsmith(talk)15:57, 7 September 2006 (UTC)
Although good articles aren't as polished as featured articles, they should at least have a few lines per topic. However, this article lacks essential information in the following sections: Optics, Hellentistic theories, The 'plenum', Particle theory revisiteds. Though not required for GA (this is just for a future FA nom), it would be good to add in-line references, external links, and a few more pictures. -- King of Heartstalk02:36, 8 April 2006 (UTC)
history
I was wondering what everyone thought of putting the "classical theories" section into a couple chunks of history - say "Before 1000 CE", "from 1000 CE to 16XX", "from 16XX to 18XX", and "from 18XX to 19XX" - or something to that effect. Listing by culture or by theory seems drawn out and not as useful as a date-sequential progression. Fresheneesz02:32, 13 April 2006 (UTC)
1st Line
Should:
I think people need light, there isnt enough light in the world. so sad. so sad. why are you so bitter?
really be here? raptor10:05, 19 September 2006 (UTC)
"He understood mathematically why a spherical mirror produces aberration."
How do you know?
light and mass
light have a mass it is very small but this is true. Can you tell me an explenation way light when passing trought sea slow down. I will tell you because of the mass of the light, without mass the light has no reasone to slow down or to reflect from object. Light whitout mass would simply pass trought any object and our ayes wouldn't see the light. Because of the mass the gravity bend the light. Without mass the light would be without any particle in it that particle is a foton.
Every person who read this comment i ask to check this with the formulas because this is not my area of expertise. And please don't delete this comment so that the other's may enjoy in little things like discovering a truth about everything.
If you can send me a mathematics proof to my email "nesmetami@net.hr". All that i tell here is true and i don't mind if some of you take a nobell prize for this discovery, i will be pleased that truth has win once again.
I thanks all the people who will work on this matter. Thank you.
Anyway, light in water does not slow down because photons slow down, but because they are continuously absorbed and re-emitted. --Army198722:45, 24 November 2006 (UTC)
in one of the first paragraphs it says Frequency (or wavelength)...who posted this article? frequency and wavelength are absolutely NOT the same thing. —Preceding unsigned comment added by 128.8.79.53 (talk)
Right, they are not the same. But you can use them interchangeably to specify the light characteristics; probably this would be more clear if not done with parentheses? Dicklyon00:06, 23 February 2007 (UTC)
Though they are interchangable, those 2 factors are critical for mathematical analysis. You can determine one from the other, but fundamentally they are different and the introduction be edited. —The preceding unsigned comment was added by 67.68.40.31 (talk) 05:08, 5 March 2007 (UTC).
Light as EM of any wavelength
Just came across this article, and the first line troubles me:
" in a technical or scientific context, electromagnetic radiation of any wavelength[1]. "
This is surely not accurate. Light is specifically EM radiation in the visible spectrum .The reference cited does not seem to back up this statement and I am curious to know who thinks it is accurate
Speaking broadly, light refers to the electromagnetic (light) spectrum
Sure seems that the reference backs up the notion in question. For the record, I've always been taught in my educated layman's physics education that, as the reference says, light can refer to the entire spectrum, and I came to this article to find out about electromagnetic radiation, not merely the visible portion (though I would not object to making this article just about the latter, if that was made clear at the top of the article).
The killer argument, though, is...well...has anyone ever heard of "the speed of electromagnetic radiation"? No. It's always the speed of light. Surely this isn't meant to only refer to what's normally thought of as the visible portion of the spectrum? Let's put this silly argument to bed. -- Calion | Talk02:32, 6 April 2007 (UTC)
It's the speed of light because visible light was originally used to determine its value. Look in a dictionary [4] and you won't find any mention of gamma rays or radio waves. The notion that the entire electromagnetic spectrum is light is perverse, and verging on the stupid. It's been brought about by arrogant scientists who think they have a right to redefine certain words to fit their blinkered thinking. Scientists, especially dim ones, do not control dictionary definitions. So radio waves are light are they? Well, can't say I've seen them - have you? 86.31.79.21521:02, 15 April 2007 (UTC)
Yes, it was scientists who redefined light for their purposes to be all wavelengths of EM. It's not because they're "perverse" or "stupid", but because a lot of science is about understanding phenomena by unifying them under a smaller number of basic descriptions. It's really not a big deal that there are both traditional and expanded definitions of light, and that sometimes you need to say "visible light" to clarify what you mean. Deal with it. Dicklyon21:07, 15 April 2007 (UTC)
This issue is also being discussed (again) on Talk:Photon. As I wrote there, I don't think "light = EM radiation" is in general use by physicists or in physics textbooks. The reference cited in the article is maintained commercial suppliers of various sources [5] and is not authoritative. I think we should look through some textbooks. Timb6612:47, 3 May 2007 (UTC)
The National Synchrotron Light Source produces "light" (per their name) from infrared through X-Ray so the idea that all (or at least, most) EM radiation is "light" is an idea that's abroad in the world of physicists.
I have looked in the 3 university physics textbooks in my office, and they all say light is a subset of EM radiation (Young & Freedman, Hecht, and Halliday, Resnick & Walker). A straw poll in the tea room (I work in a physics department) gave the same response (although one person had the other view). Despite the interesting counter-example of the National Synchrotron Light Source (maybe they wanted a catchy name?), I still think most physicists view light as a subset of EM. Are there any other couter-examples out there? Timb6606:52, 8 May 2007 (UTC)
The first book I came to, Max Planck's 1930 second edition, 1932 English translation, of Theory of Light, says, "...whereas the most important optical concepts, those of light and color, were originally derived from the impression on our eyes, these concepts have nothing at all to do with the immediate sensation of sight in present-day physics—a trend of development which has justified itself in the abundant fruit which it has borne." Interpret that as you like. I find nothing else in the whole book to suggest any connection of the book's main topic to human vision. Dicklyon07:04, 8 May 2007 (UTC)
Here's another book from my shelf: Millikan & Mills, 1908, Electricity, Sound, and Light. In this one, I can't find anything that says. They don't mention IR, UV, X-Ray, or Gamma, and what they say about EM radio waves is not much more than that they travel at the velocity of light. He cites spectral lines out to about 0.7628 micro-meters, which is pretty much invisible IR, I think. But basically he never says anything that would help define the scope of "light". Dicklyon07:27, 8 May 2007 (UTC)
Anohter: Millikan & Gale, 1927, Elements of Physics, definitely uses "light" only for the visible rays. Dicklyon07:34, 8 May 2007 (UTC)
And Oldenberg, 1949, Introduction to Atomic Physics, refers to "the incident light" in talking about Compton's X-Ray experiment. Also refers to ultraviolet and infrared as light, sometimes, and other places says "in the ultraviolet" without saying light. So, mixed. No clear definition is in force. Dicklyon08:09, 8 May 2007 (UTC)
You know, the way we speak about things actually does change over time, and a reference from 99 years ago really isn't all that useful today; a couple or three things have been discovered in physics since that time. I will also note that it's not just the American synchrotron specialists who like the idea that "light" can be of quite a variety of wavelengths. The folks at the European Synchrotron Light Source[6] seem to feel the same way as do the folks at lightsources.org.
Yes, of course. It's just that the shelf at hand has old books, and the historical roots of usage are also of interest. I think the Planck book actually is the more modern in terms of approach and terminology, and I'll look for more refs as I get a chance. Dicklyon14:58, 8 May 2007 (UTC)
Nice discussion but I think it causes quite an important fact missing from the article: What is the wavelenght of (visible) light? Speaking German helps with that question (the German article is much more what one would expect in this regard). Looking up "electromagnetic spectrum" would peobably also help but I guess most users know neither German nor where else to look. Should IMHO be mentioned in the introduction as it is a fundamental attribute of light. 192.160.142.311:36, 24 August 2007 (UTC)
Against. Light is just one part of the EM specturm. Contrary to what some opinionated scientists have to say, the whole EM spectrum is not light. As any sane person knows, only the visible portion is (well maybe UV and IR as well - just). 86.31.79.21521:04, 15 April 2007 (UTC)
Oppose per 86.31.79.215. Also, who is going to look for electromagnetic radiation whe they want to know about light? Certainly, an informed person on the subject might, but this is an encyclopedia for everyone, and the term 'light' is still scientifically accurate. Plus, I think there is just too much content to simply be merged. I like the article, and I think that to see a large, good article on something like light be merged would be sad, because the topic of light needs much coverage, something i think it can best have as it's own article. ---ÅñôñÿMôúsDîššíd3nt05:23, 17 April 2007 (UTC)
Wave theory
As waves are not affected by gravity, it was assumed that they slowed down upon entering a denser medium
One thing has become clear to me in my year with wikipedia: writing stuff without sources is time wasted. Your work will be eroded by people who think differently, or who miss the point. But if you attribute your writing to a verifiable source, it is likely to last and be defended against changes that make it disagree with the source. This article has come a very long way with no sources to speak of. Hopefully some of you will take a hint and start helping with that. One way is to just remove anything you don't like, and challenge whoever puts it back to cite a source. Of course, if you do that too much you'll piss someone off; still it's officially fair to remove anything that's unsourced, and often it just needs to be done. Dicklyon06:14, 8 May 2007 (UTC)
Sorry, I meant to put a comment, Philosophus. The photon analogy was good there in the Indian concept, if not perfect, so I put it back. Dicklyon19:21, 27 May 2007 (UTC)
Precise nature of light is not at question
Saying "The precise nature of light is one of the key questions of modern physics." implies there is something important missing from our understanding of light. Without reliable references, I would dispute that. We understand light very well. The study of light is, of course, still important, hence my change to the wording "The study of light, known as optics, is an important research area in modern physics." Timb6605:13, 8 June 2007 (UTC)
"The precise nature of light" is here standing in for the larger problems in quantum physics, namely how to interpret the wave and particle behaviors; the Copenhagen Interpretation is widely regarded as falling short of this goal. Here's a book with some info on this thorny question. I've just been reading The Trouble with Physics by Smolin, which points out the some current theories also have light traveling at variable speeds, and stuff like that. Other interpretations, such as the Mead's Collective Electrodynamics, explain the so-called particle aspects of light purely in terms of waves; particle aspects are an epiphenomon, not part of the precise nature of light, in that theory.
I think it's a bit parochial to think that the "precise nature of light" is understood. The change to say that optics is important trivialized what was, in my opinion, an important part of the story about light. I'll see if I can find you a better reference.
The speed of light in a vacuum is exactly 299 792 458 m/s (fixed by definition).
Is this fixed by definition? The speed of light is neither an arbitrarily defined constant nor is it arbitrarily defined as fixed. The units are, of course, arbitrary definitions and so the term applies to them and their definitions affect the value of the constant, but I don't believe the term applies to the speed of light itself. Ros070908:23, 14 June 2007 (UTC)
Although this quantity is sometimes referred to as the "velocity of light", the word velocity is usually reserved for vector quantities, which have a direction.
Indeed. And I have never heard it called the velocity of light - is this some geographical difference? The term velocity is rarely used in common conversation, indeed speed is often used when in fact velocity would be more scientifically accurate. That is to say, the only people likely to say velocity instead of speed are probably scientists, who would know it was the wrong term so they wouldn't use it. Ros070908:23, 14 June 2007 (UTC)
An earlier note on this page suggests that Einstein used the term velocity; if so this counters my argument that a scientist would not do so! Ros070908:28, 14 June 2007 (UTC)
The word "velocity" is commonly used in place of "speed." Reasoning that just becuase Einstein used it in common verbage is not a good argument that it is the correct term to use here. In physics it is unambiguosly (sp?) clear that "speed" is a scalar and "velocity" is a vector quantity. You could actually speak of the velocity of light if you were referring to it with a direction. (vector quantity) BUT, in this context speed is the correct word to use, assuming, that we want the article to be scientifiically correct. Also the speed of light is fixed by definition. Refer to this : http://en.wikipedia.org/wiki/Speed_of_lightStarkrm16:30, 14 June 2007 (UTC)
On the velocity/speed issue we are absolutely agreed that difference is relevant; my point was that I have not experienced people using the term incorrectly - the common "phrase" is "speed of light" not "velocity of light" - and if people are not using the term incorrectly then there's no point making the correction. However, my own experience is limited (as is everyone else's - hence my comment about geographical differences) and if the term is misused then the clarification is useful. My follow-up comment about Einstein's usage was not to reinforce my comment but to refute it after I gave it further thought; I think that may not have been read as intended. On the "fixed by definition" issue I defer: the reference you provided makes it clear. Ros070917:29, 14 June 2007 (UTC)
Order and Percentage of Light Sources
Hi. What is the percentage by order of light provided by natural sources during the course of a diel? I assume the Sun, the Moon, the Stars and other occasional phenomena are the order of environmental contribution, but what are they in terms of percentage of contribution to a 24-hour period? thanks Stevenmitchell (talk) 00:33, 3 January 2008 (UTC)
Bending (reflection) focusing (refraction) and stopping (gnomon for example). language specifically used in young persons workshops on light with the aim of increasing their understanding.
Like many other natural events people have made light a tool but unless I missed it it is not in the article anywhere , and could there not be examples of the uses of light by people. This could be expanded into artist and scientific uses of light. Your thoughts? Edmund Patrick ( conferwork) 21:18, 2 February 2008 (UTC)
Romers article here says that he calculated it took 20 minutes for light to travel from the sun to Io, but the Speed of Light section says thats the time (With a rough 2 minute difference) he said it took to travel around the Earth. Heck, the same section (The light one, not Romers) says the circumference of the Earth was not known back then, and I seriously doubt this wise astronomer was such an idiot he would make such an assumption.
Can we fix this? Or am I just misreading it? I'm trying to do research on light physics so I can do proper experimentation with light and I would like the background info to go off of. —Preceding unsigned comment added by 74.14.89.26 (talk) 18:21, 16 August 2008 (UTC)
Two Comments on the Article, "Light"
At the empirical level, the article fails to mention the source of all light. According to the most widely accepted theory of the universe - The Big Bang theory - all light began at the time of the Big Bang. Alpher, Bethe and Gamow were credited with the idea. Gamow and Alpher were at George Washington University at the time; Bethe, already an established physicist, was added as an author of the original 1948 paper by Gamow, who had an outrageous sense of humor.
At the "spiritual" level, the article fails to recognize that Isaac Newton, the scientist recognized by many as the Father of Empiricism, had a distinct religion himself. One can find out more about Newton's beliefs by simply reading his work in the original. —Preceding unsigned comment added by 98.242.94.5 (talk) 21:58, 24 September 2008 (UTC)
Ptolemy
"This contradicted Ptolemy's theory of vision that objects are seen by rays of light emanating from the eyes."
How did Ptolemy then explain darkness, or that looking directly into the sun is damaging the eye whereas looking at the moon isn't? And that in rooms without a window and any kind of fire, candle or lamp you can't see a thing? —Preceding unsigned comment added by 80.141.192.84 (talk) 15:56, 6 October 2008 (UTC)
Reflection
There should defnitely be a section on reflection in this article.
In the longer term, a page on reflection should be created.
Contrary to what's stated in the other topic, I was taught light itself is not visible to the human eye. What we see is the influence of light on other objects or particles, according to the wavelengths they absorb and reflect, but light itself is never visible. When a "beam of light" appears to be visible (like in the prism picture at the start of the article), what's actually seen is a stream of illuminated particles. Dust, for instance.
If "visible" is defined as "detectable by means of an organ called "eye"", then light can be visible. Of course your beam of light is not visible (unless it directly hits your eye), but the light scattered by the illuminated particles is perfectly visible - by definition. DVdm (talk) 16:51, 24 November 2008 (UTC)
My Question is : Why Far Lights , like when we are on the coast, and in da Other side , there a light source , a ship or any shining object .. we see the light as a discontinuous light , like a slow Stroboscope . why that ? is it related to the power and intensity of the light ? or related to the frequency ? —Preceding unsigned comment added by 217.151.224.29 (talk • contribs) 11:29, 14 December 2008
Most likely the result of terrestrial scintillation, an atmospheric phenomenon similar to that which causes the apparent twinkling of stars. Light from distant objects near the horizon must travel through an especially great thickness of air, which accentuates the effect. Hertz1888 (talk) 14:28, 14 December 2008 (UTC)
Hindu Theory
Within the Hindu Theory of light reads the sentence:
Around the first century BC, the Vishnu Purana correctly refers to sunlight as the "the seven rays of the sun".
I presume that the "seven rays" refers to the seven colors of the spectrum of white light, no douubt. I do object to the use of the word "correctly" within the sentence. That we use seven color names to describe the range of wavelengths within white light is not based on some natural physical model or process, but is a matter of convience.
There is no physical propcess which gives rise to exactly seven distinct colors and it is a matter of modern convention to use seven colors: Five colors would have served just as well and three colors would be more aptly called "correct" since the physical process that defines color recongnizes only three colors, all other colors being a combinati9on of those three.
So while the philosopher may have poetically described light as "seven rays" coincident with modern convention, there is nothing scientifically "correct" about that description that is not otherwise correct with a different number, unless you are talking about how he correctly described modetrn convention, which I doubt very much and is irrelevant anyway.
Remove the word "correctly" from the sentence and the paragraph regains the credibility it otherwise deserves.
05:25, 22 December 2008 (UTC) —Preceding unsigned comment added by Radzewicz (talk • contribs)
Speed of light discussion confusing?
I found the discussion of the speed of light confusing. In particular the section:
Light always travels at a constant speed, even between particles of a substance through which it is shining. Photons excite the adjoining particles that in turn transfer the energy to the neighbor. This may appear to slow the beam down through its trajectory in realtime. The time lost between entry and exit accounts to the displacement of energy through the substance between each particle that is excited.
By "the neighbor", do we mean "their neighbors"?
It says light "appears" to slow down, as though it does not slow down in reality. Does it slow down or not?
What does "realtime" mean? I know the definition in software, but I don't know how to apply it in physics.
Perhaps this paragraph is trying to indicate that the photon covers more distance than it would have in a vacuum? If so saying this explicitly would be helpful.
What medium does light use to travel across space? What is light in terms of atoms? Could it be the vibrations of electrons or something? —Preceding unsigned comment added by 86.135.98.182 (talk) 01:05, 9 April 2009 (UTC)
Actually, light itself is invisible. If it wasn't you wouldn't be able to see anything, because the light would blind you. Serendipodous08:12, 18 May 2009 (UTC)
This is an archive of past discussions about Light. Do not edit the contents of this page. If you wish to start a new discussion or revive an old one, please do so on the current talk page.
Making a distinction between EM radiation and light seems a bit POVish to me. The only purpose seems to be to make a distinction between light and visible light. But there already is an article on visible light. Yes there are a number of cultural elements that don't really belong in a scientific article about light but these could be merged with visible light. Serendipodous08:12, 18 May 2009 (UTC)
The book "Campbell, Biology" also states that the (for human beings) visible light ranges from 380 to 750 nanometers, the German Wikipedia entry says 380 to 780 nanometers, though, but it does not cite its source, unlike the English article here. Can additional sources be added for this claim, at best scientific research or similar? 80.108.103.172 (talk) 17:24, 6 July 2009 (UTC)
People of different ages can see somewhat different colors of light. As people get older, they generally loose acuity to colors of the deepest blue and indigo, and to the the reds with the longer wavelengths or lower frequencies. Remember also, that for all electromagnetic radiation, (the wavelength)x(frequency) = the speed of light in the physical medium.98.81.17.215 (talk) 00:16, 19 March 2010 (UTC)
"Repulsive aspect"
I removed the following section entitled "Repulsive Force"
Repulsive Force was discovered by team of Yale scientist by splitting infrared light into two beams so that each travel on a different length of silicon photonic nanowire, called a waveguide. The two light beams became out of phase with one another, creating a push, or repulsive force, with an intensity that can be controlled; the more out of phase the two light beams, the stronger the force. These forces are tunable forces that can be manipulated.[1]
because:
1. The "reference" is very un-scholarly and does not cite the original research paper
2. The entire section was unclear and ambiguous: a repulsive force between what and what , and under what conditions? The "reference" was also very unclear and devoid of proper physics.
Medieval authors take for granted the distinction lux/lumen, something which should be mentioned and, if possible, explained. Translations most often chose to ignore it. 91.92.179.172 (talk) 21:33, 25 September 2009 (UTC)
The to-do list above seems to me to be mostly irrelevant for this article. specifically:
The beginning section should summarize the main topics of the article, including, but not only, those mentioned.
Light in practice - mixes different subjects:
Sources of light are already described. Types of light-bulbs should be in a separate article.
A laser is a different topic and should be given a section.
visible vs infrared should be mentioned in wavelengths and introduction.
Properties of light are an important section, but should contain a different topic list.
Light in the Universe - no.
Light in everyday life - no.
The eye (vision) - yes, very important, as a separate section with another name.
The effect of daylight on human senses - could be covered under the eye, anyway the gangelion cells in the eye have that function.
Light in industry - no. Mainly because there is no clear separation between light in industry, in communication, in technology, at home. instead:
Uses of light should have a separate section. That includes solar cells and power plants, and optical communication.
Light in science - yes, a reasonably well-defined topic. Could also be inside uses of light. The topics are good too.
Light in theory - should be given in lay-person terms, they are given in the history section but should be also given as a full comprehensive review. However, Detailed description of light in physics should be kept in photon and electromagnetic radiation.
The history of light and light-speed is well done, not to-do.
Does anyone object to me setting up automatic archiving for this page using MiszaBot? Unless otherwise agreed, I would set it to archive threads that have been inactive for 30 days and keep ten threads.--Oneiros (talk) 13:37, 24 January 2010 (UTC)
These two statements, from the "Speed of Light" section, seem contradictory:
1. "Light always travels at a constant speed, even between particles of a substance through which it is shining."
2. "Two independent teams of physicists were able to bring light to a complete standstill by passing it through a Bose-Einstein Condensate of the element rubidium..." —Preceding unsigned comment added by Fslap (talk • contribs) 16:44, 13 August 2009 (UTC)
The phrase "complete standstill" is quite an exaggeration. Electromagnetic radiation never stops completely, but rather, in certain substances, it can approach an approximate speed of
All of them. Colors are a property of light defined by wavelength, so all colours are carried by light and only by light.
If you mean "in white light" or "in sunlight", the answer is the same, because sunlight is white light and it is a broad-spectrum source, which carries all the frequencies of visible light, and therefore all the colors. Setreset (talk) 11:02, 9 March 2010 (UTC)
Light sources
Photo in Light sources section does not seem to support the text. Caption states that it is a photo of a cloud illuminated by sunlight. Though the caption may be correct, the illuminated portion of the cloud is not shown with the exception of the lining. In fact, the cloud obscures the sunlight. It would seem that a photo actually showing a light source referenced in the section would be more appropriate.
Brookeok (talk) 03:28, 9 September 2009 (UTC)
The phase velocity of light never changes. It is always = c.
The group velocity can be changed freely and easily producing the stupid effects popular with people like EIT and shining light through a BEC for whatever reason. —Preceding unsigned comment added by 134.154.212.74 (talk) 18:21, 3 June 2010 (UTC)
Reflection, defraction
This might just be me, but i cant seem to find anything on the relflction of defraction of light or wavelengths in there. does it need adding? --Conspiracy Theorist (talk) 07:30, 24 April 2010 (UTC)
This Article is Terrible- June 3 2010
This article needs to be almost entirely rewritten. The history section is too long and probably too completely inaccurate. In fact, the whole article doesn't read like a science article as all. It's full of holes everywhere. Which few sections that are neither out-of-place or terrible are in the wrong order.
You could argue that real science should be left for a good article like "Electromagnetic Radiation." Light and Electromagnetic radiation are really two different concepts. Likewise, Optics and Electromagnetic Theory are two relatively different fields of study. This article could be good too.
When people are searching for "Light" they probably want to know:
What it is. Light is one type of propagating electromagetic radiation. It is created when free charges are moved. This happens in stars, florescence, etc.
The history of light in science. No one needs to know that the Bible says that light is emitted from the Moon or that the Mayans worshiped the sun or equal crap. This is a science article. Everything else can go in its own article.
The Physics of light. The optics section is currently two fucking lines long. Light is the most important object in our current study of the universe. Without light we wouldn't have any understanding of cosmology, optics, electromagnetic theory, or atomism. We should show it respect here.
Other nonsense that appears at the bottom of wikipedia articles. Here is where all the inane bullshit goes. "The sun produces too much light in the third episode of arrested development causing Michael to block his eyes with his hand."
And don't ask me to fix the article. By now you can probably guess how long I can be on wikipedia without pissing off some ignorant moderator or administrator. Not very long.—Preceding unsigned comment added by 134.154.212.74 (talk • contribs)
By Wikipedia's nature we are all partly responsible for the shape of this article, but I think that those who are most responsible for it, are the ones who really know that and why it is in bad shape, but who don't want to be asked to fix it. DVdm (talk) 18:26, 3 June 2010 (UTC)
Well, I'm scared to. I mean, it took me about a minute and a half to figure out how to respond to your comment. If I ever learn how to edit wikis (I'll probably start with a less serious one), I'll spend a little time editing a few science articles. —Preceding unsigned comment added by 134.154.212.74 (talk) 18:59, 3 June 2010 (UTC)
Good idea. When you write on talk pages (like this), please don't forget to sign your messages with the four tildes. There a click-link under the edit window. Good luck. DVdm (talk) 19:27, 3 June 2010 (UTC)
To the anonymous IP user (134.154.212.74). Thanks for the feedback. I hope you do edit science articles on Wikipedia. I am sure you are a welcome addition. First, let me say that editing on Wikipedia is as easy as typing on a keyboard. And don't worry about making mistakes, such as typos or spelling. Someone will come along and correct it. Especially with physics articles - people whip out there particular automated editor and voila! Also, there are others who prefer to come through articles and do correctionsmanually.
Second, you may be interested in Wikiproject physics where this WikiProject exists to improve the quality of existing articles related to physics, and to create articles to cover a broader range of physics topics. To help with these and other goals, there is a talk page where editors interested in Physics discuss various issues: Wikipedia talk:WikiProject Physics. Maybe you could take a look. ---- Steve Quinn (talk) 02:48, 31 August 2010 (UTC)
Third, personally I think this article fits the definition of an encyclopedia article very well. True it is a science article, but it is also meant for the general reader. I have only done a quick overview of the later sections of the article. However, discussing the different views of light that people once had, and the different theories of light throughout history seems appropriate and relevant, because this is most likely of interest to the general reader.---- Steve Quinn (talk) 03:06, 31 August 2010 (UTC)
Edit history error
I entered an incorrect comment in the edit history using the AWB, here [7]. It was supposed to be AWB - typo fixing. The correction was changing the formats of the dates, for the date parameter in the citation template. ---- Steve Quinn (talk) 18:33, 29 August 2010 (UTC)
This article appears to be fully referenced to me. It is referenced from the introduction to almost the last paragraph. The article also has links to larger, main articles, and references in these areas is not so critical. I really do not see the problem with this article. ---- Steve Quinn (talk) 17:21, 12 September 2010 (UTC)
Alhazen
Alhazen is only mentioned as being reject by Descartes. This guy proved that light moved in rays and reflected and refracted. He needs to be included here. On that note, there needs to be separate articles. One for the actual science on light and another on the history of the understanding of light.
As well, to illustrate my point, here is part of the article on alhazen.
"Ibn al-Haytham's most famous work is his seven volume Arabic treatise on optics, Kitab al-Manazir (Book of Optics), written from 1011 to 1021. It has been ranked alongside Isaac Newton's Philosophiae Naturalis Principia Mathematica as one of the most influential books in physics[40] for introducing an early scientific method, and for initiating a revolution in optics[41] and visual perception." —Preceding unsigned comment added by 68.36.55.23 (talk) 10:41, 12 September 2010 (UTC)
In the last 100 edits to "Light" there are not more than 6-8 true edits. The others are all vandalize-undo sequences. Is it time to semiprotect the page? --GianniG46 (talk) 14:01, 18 November 2010 (UTC)
It appears that the light wave graphics is backwards. Light waves propagate in the direction of E x B (the direction of the Poynting Vector), so a light wave propagating to the right should have positive values for E wherever it has positive values for B. Unless the diagram in the article is using a left handed coordinate system or some other oddity, it is incorrect.
76.104.101.94 (talk) 05:12, 19 November 2010 (UTC)
Another problem with the image (that I also see in many images on the internet) is that the E and B fields are spatially in phase with each other. If I work through Maxwell's equations for the case of a plane wave polarized along the x-axis (and traveling along the z-axis), I find that the magnetic field is a plane wave polarized along the y-axis and whose spatial and temporal components are out of phase with the electric field. Since we are looking at a snapshot we can fix the time at a particular value and just look at the spatial variation: If the electric field has sine variation along the optical axis, then the B field should have cosine variation. 67.171.217.43 (talk) 16:52, 8 January 2011 (UTC)
Straw and refraction
The text under the picture of a straw in a glass with bluish liquid says:
"An example of refration of light. The straw appears bent, because of refraction of light as it enters liquid from air."
I am pretty sure that straw really IS bent, and that that has nothing to do with refraction refraction.
I guess the picture is supposed to show that the straw seems to be bent at the liquid surface, but it doesn't. —Preceding unsigned comment added by 194.237.142.17 (talk) 08:41, 8 December 2010 (UTC)
Edit request from Agentmarmite, 18 February 2011
{{edit semi-protected}}
RE: an important missing physical theory of light.
A paper, titled "Light: The Rope Hypothesis", has recently been published in the Proceedings of the 2010 International Conference on Physics, Science and Technology. The reference is:
Gaede, Bill, "Light: The Rope Hypothesis," 2010 Conference on Physics, Science and Technology (ICPST 2010), Hong Kong, China, Dec. 28-30, 2010, Ed. Jiang Hui, pp. 63-66, (ISBN978-1-4244-9124-7, IEEE 2010). The ICPST 2010 proceedings are published by IEEE and papers will be archived in the IEEE Xplore and Indexed by Ei Compendex and ISI.
IEEE Catalog Number: CFP1098L-DVD
ISBN: 978-1-4244-9124-7
The website is at: http://www.icpst.org
The Rope Model of light and the atom has also been published in:
Gaede, William, "Light: neither particle nor transverse wave", in "What is the Electron?", Ed. V. Simulik, Apeiron (2005).
Gaede, Bill, "Why God Doesn't Exist," ViNi (2008).
Yeah -- what is this rope hypothesis? What sort of information is supposed to be added to the article? Please post that then repost your edit requested template. :) Banaticus (talk) 06:57, 19 February 2011 (UTC)
Agreed, I will gather secondary sources & add a required elucidation/summary of the Rope Hypothesis & its relevant particulars from the peer-reviewed papers. Agentmarmite (talk) 22:21, 19 February 2011 (UTC)
Light wave graphics
You have the graphics wrong on EM pulse the M wave sb 180 degrees out of phase to the E wave.
The M wave propagates the E wave only if they are 180 degrees out of phase.
Move the M wave forward (or back) 180 degrees — Preceding unsigned comment added by Memphiseddy (talk • contribs) 22:21, 21 February 2011 (UTC)
Edgar, it would be good to roll back your edit, discuss the intended direction here first, and then do it right. Several problems I see right off the bat: your intro sentence is written about the term, light, rather than about light itself; this is almost never a good way to start. Second, you don't seem to be experienced enough to know or copy some of the basics of style, like that we use sentence case for headings. The rest I haven't reviewed, but it would be better to tell the many editors of this page what direction you think would be an improvement, rather than characterizing their work as "a mess" and throwing it out for your own mess. Dicklyon (talk) 18:25, 10 April 2011 (UTC)
Currently reference number two is inconsistent with the third sentence in the introduction. The statement in the introduction reads, "In physics, the term light often comprises the adjacent radiation regions of infrared (at lower frequencies) and ultraviolet (at higher), not visible to the human eye.". Reference number two is as follows: Gregory Hallock Smith (2006), Camera lenses: from box camera to digital, SPIE Press, p. 4, ISBN9780819460936.
The reference actually says: "In an extended definition of light all wavelengths throughout the electromagnetic spectrum would be included..." Then in the next paragraph it reads, " Scientists and engineers now use all these wave bands for numerous applications ranging from radio transmission to x-ray. Optical engineers are interested in wavelengths throughout the ultra-violet, visible, and infrared regions, and occasionally even x-rays"
So this statement probably needs to be corrected. This is because, the source doesn't actually define the term "light" for physics as currently stated in the article. In fact, the source may indicate otherwise. However, the source only refers to scientists and engineers, and this group uses all these wave bands (in the electromagnetic spectrum). Furthermore, according to this source, it is only optical engineers who are interested in a more limited spectrum of light.
Reference number three appears to deal with, directly, only the photoelectric effect, and nothing really related to the introduction. Obviously from the title, this book has broad topical coverage, but the reference is linked only to the photoelectric effect. This reference is as follows: Narinder Kumar (2008), Comprehensive Physics XII, Laxmi Publications, p. 1416, ISBN9788170085928.
I think this sentence use to say that when physicist use the term "light" it can mean the entire electromagnetic spectrum (or something like that). I think a sentence that says this, or one that says scientists and engineers, is important in order to give an accurate (or unbiased) view of how the term light is used, and even perceived. Any comments? ---- Steve Quinn (talk) 04:31, 18 January 2011 (UTC)
I think the most important thing is to be very clear that definitions vary; cite several sources, or one that actually says that (maybe this one). See a variety of conflicting definitions in other books. Dicklyon (talk) 05:34, 18 January 2011 (UTC)
Dicklyon, these sources are very interesting. I have looked at only two sources so far, the links to "Outdoor Lighting: Physics, Vision and Perception", and "Atoms, molecules and lasers". After only briefly reading these two, I agree that the most important thing is to be very clear that definitions vary. Thanks for pointing this out, and thanks for the links to the books (very helpful) - I look forward to reading the relevant pages. Furthermore, it appears that a section within this article could be written pertaining to this topic. ---- Steve Quinn (talk) 21:58, 18 January 2011 (UTC)
I am still going through sources, as time permits. Just for the heck of it, I decided to check out other encyclopedic sources (or other encyclopedias) regarding this topic. All of them define "light" as that portion of the electromagnetic spectrum that can be detected by the human eye (at approx. 400 to 700 nanometers), and electromagnetic radiations just on either side of visible light. There are variations on the wording. The only one that hasn't used this broader definition is Encyclopedia Britannica online, which essentially defines only the visible spectrum as light [8]. The other encyclopedic sources are Encyclopedia Americana {Grolier Online) , Grolier Multimedia Encyclopedia, and Gale Science In Context (which appears to have more than one relevant article). For these other sources, besides Britannica, I need to use an account. If anyone else has an account, I have some notes on one of my sub pages with links here [9]. ---- Steve Quinn (talk) 04:56, 24 January 2011 (UTC)
The article uses both "color" and "colour", "behavior" and 'behaviour" and also "localise" while using "standardize", "polarize" and "summarize" elswhere. I would suggest unifying the style to the American one, since that seems to be prevalent throughout the article. I do not have the necessary permissions. - 109.233.64.81 (talk) 22:40, 3 June 2011 (UTC)
Guidance is to be found at WP:ENGVAR, particularly in its subsection at WP:RETAIN. However, the tag at top of this talk page indicates British English has been adopted. A broad consensus would be needed to change to the American variety. Lacking that, it would appear prudent to make the article consistent in U.K. style. The overriding principle in the manual of style is internal consistency. Hertz1888 (talk) 23:09, 3 June 2011 (UTC)
Thanks for your work on this. The analysis could, of course, conceivably indicate the extent to which the article had drifted away from a previous prevalence the other way. An examination of the page history would be needed to establish whether that was indeed the case. Hertz1888 (talk) 02:03, 4 June 2011 (UTC)
Incorrect Graphic?
In the "Electromagnetic theory" section, the graphic shows the two wave components in phase. I believe they should be off by 90 degrees, so when one is at its full height, the other is at zero. ABlake (talk) 14:13, 8 July 2011 (UTC)
"René Descartes (1596–1650) held that light was a mechanical property of the luminous body, rejecting the "forms" of Ibn al-Haytham and Witelo as well as the "species" of Bacon,"...
Done That's a good one. Thanks for catching it. New links should be tested when added, but all too often that's not done. Hertz1888 (talk) 03:33, 25 July 2011 (UTC)
You've proposed this before, a couple times. I'm unsure about this. Admittedly, the separation made more sense at one time than it does now. Some time ago, this article was about light in the physics sense of the term (redundant with electromagnetic radiation), and material specific to visible light was in what is now visible spectrum (the name of the article changed at some point). If this article is going to be just about visible light, perhaps it makes sense to discuss the spectrum here, replacing the current section Electromagnetic spectrum, which is no longer appropriate. On the other hand, there is probably too much material to merge it all into one article. We should think about the most sensible way to divide the material between two or more articles. The current division might well be the best we can do.--Srleffler (talk) 00:23, 16 July 2011 (UTC)
I don't think a merge is a good idea. These appear to be two distinct articles. Contrary to the proposal, I hardly see any overlap. I think the first few lines of each article start out very similar, but very quickly they each take off into different directions.
This article is appropriately all about "Light" and the different ways its interactions are viewed (for lack of better terminology at the moment). For example "speed of light" is one view, "refraction" is another view, "light sources" is another relevant discussion of the term "light", and "optics" is appropriately mentioned. Particle theory (of light), Wave theory, Electromagnetic theory, the special theory of relativity, quantum theory, etc., etc. are other views and discussions of light
Other relevant topics might also be here: [10]. It could be how light seems relevant to people is here. And this article provides similar coverage to that last link. This how a well known encyclopedia covers "light" [11]
I am sure "light" is discussed much more than the "visible spectrum" and this is a general encyclopedia, not a science encyclopedia. I think, to try to fit what is in this article into "visible spectrum" would be cumbersome. Both the general reader, and one with a scientific background can relate to the term "light". Furthermore, we may be creating some kind of exclusivity by merging all this into one article entitled "visible spectrum". It might be best to realize that the term "light" has probably been in the general and scientific lexicon much longer than the term "visible spectrum".
Finally, "visible spectrum" appropriately covers the smaller slice that is the scientific view. If there must be some kind of merge then I think "visible spectrum" could be several sections of this article. Also, "visible spectrum" is a relatively short article. ---- Steve Quinn (talk) 02:58, 16 July 2011 (UTC)
I support merging because wikipedia should be more objective than subjective. Visible spectrum and Light are the same from the objective point of view. --Wayiran (talk) 23:54, 3 September 2011 (UTC)
Relationship between additive and subtractive color mixing of light.
Light can be mixed additively as in combining several luminous sources and subtractively like the mixing of paint. Both methods of mixing light follow their particular set of rules, but those rules have always existed in isolation -- one set for light and another for paint. The image below visually states the possible relationship between additive and subtractive color mixing.
To answer my own question, I found a short notice about unproductive edits in the talk archive from November 2010; semi-protection obtained as a result. Time to reconsider, I reckon. Martinor (talk) 06:02, 19 August 2011 (UTC)hgfgfghjdfhdhgfdhfggg123456789q3456789oiujhgfdsaghbjbhkujghuit6rftyhfgvhggkjhgyu
book to add
Albert Einstein didn’t prove that the photon exists. He did theorize the quantum effect and received the Nobel Prize in 1921 for the photoelectric effect. Arthur Compton did the experiment to prove that the photon exists in 1923. In 1927 is when he received the Nobel Prize for proving the existence of the photon. Read the Nobel Prize paper in which Arthur Compton mentions for the first time the photon as a particle and the experiment that proves it.
The book
X-rays and electrons
An outline of recent X-ray theory
By idk who this is
Arthur H. Compton Ph. D.
Copyright 1926
By D. Van Nostrand Company
This book includes papers from 1923 on and uses the word photoelectrons for the one particle and electrons for the other. He shortened the word photoelectrons to photon by eliminating electro. The new particles thus became the photons from a shortening of photoelectrons. Read the book.
The term’s used for electrons from the many papers are:
• photoelectrons
• recoil electrons
• beta rays(OOOOOOIUUUUUUUUUUUUUUJJJJJJJJJJJJJJMMMMMMMMMMMKHNJ)
The term’s used for photons from the many papers are:
• x-rays
• x-ray quantum
• light year does not exist
• light darts
• electromagnetic waves
• radiation
• radiation quanta
• radiation quantum
• quantum
He used photoelectrons a lot for the electrons but for the photons there was no common term.
The photon is the particle form of light.
68.171.143.254 (talk) 17:25, 2 September 2011 (UTC)
Only humans see light?
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I would like to edit the beginning of this page because it says that light is visible to the human eye, but has no mention of animals ability to see light too.
You are a registered user so you should be able to make the change yourself. Maybe you will need to wait a few days, I am not sure, and maybe you will need to make some more edits elsewhere.
Not done: That's right. Four days and ten edits will make you autoconfirmed. You are at eleven days and nine edits. Just make one more edit and you will be able to edit semi-protected pages. Welcome, Celestra (talk) 15:13, 7 February 2012 (UTC)
Edit request on 22 February 2012
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light is ".. responsible for the sense of sight" (?) horrible!
The statement is both technically incorrect and a gross stylistic blunder.
Light, taken as a whole, can not be "responsible" for a feature of the human organism, by any ordinary definition of responsibility.
Visible light is the physical stimulus for the sense of sight.
Not done: please establish a consensus for this alteration before using the {{edit semi-protected}} template. Taken in context, the text doesn't seem obviously incorrect. You may find it easier to reach a consensus if you present the old text and the suggested text and explain how yours is better, rather than attacking the existing text and, by extention, the editors who wrote it. Thanks, Celestra (talk) 15:39, 22 February 2012 (UTC)
Light v EM radiation
Am I the only person who thinks that this article and the Electromagnetic radiation are in a complete mess. The lead of this article says it's about visible light, yet the body of the article is talking about light at any wavelength or, more generally, EM radiation.
Shouldn't pretty much everything in this article be merged with the EM radiation article so that this article can focus on visible light? GDallimore (Talk) 15:49, 5 April 2012 (UTC)
Looking at past discussions, the chance are that this proposal with just disintegrate into acrimonious accusations of POV-pushing about whether "light" means "visible light" or "electromagnetic radiation" in general. My personal view, as a physics tutor, is that I use light to mean EM radiation in general, but I accept that, in general terminology as well as in the view of certain standards bodies, "light" more normally means "visible light". So, I am putting my personal POV aside in this discussion.
Here is my proposal: this article should focus solely on visible light. This will make it a very short article compared to its present form since most of its current content would be better discussed in the article on electromagnetic radiation. Some obvious examples of content to be moved to the EM article are the sections about the history of wave-particle duality and special relativity. Although there's perhaps an argument that historic experiments made using visible light should at least be mentioned here even if a full discussion would be better placed in the EM article.
Whatever other people think about how to perform a merge, I hope everyone can agree that the present situation of two overlapping articles is totally untenable. GDallimore (Talk) 16:08, 5 April 2012 (UTC)
COMMENT You're not really suggesting a merge. I think a true merge to one giant article has a WP:SNOWBALL's chance, as the subjects of light and EM radiation are both very large ones, even if we restrict "light" to visible light and redirect it that way, and make it a main article for visible light in EMR. Per WP:SS, we simply need the room for both articles, and overlap must be kept to a minimum by editorial choice.
Restricting light to visible light is also my own preference, but that's not a merge, rather it involves redirects and movement of some material. One can have a redirect from light to a dab page, or else we can make "light (visible)" the primary redirect and dab to other uses. Light may refer generally to any EMR (although when we say light we usually don't mean radio waves), so long as a qualifier is put in, as in "UV light." Use of "light" with no qualifier has heavy connotations of visible light, and the encyclopedia should be constructed that way.SBHarris19:08, 5 April 2012 (UTC)
It's the closest wikipedia has to what I think needs doing, which is salvaging useful material from this article and putting it in the other one. GDallimore (Talk) 22:47, 5 April 2012 (UTC)
Actually, no, my original proposal is correct. I am proposing the merger of some SECTIONS of the article into the other article. It's a merge proposal, no ifs, no buts. GDallimore (Talk) 11:20, 7 April 2012 (UTC)
You made a "merge proposal" that violates WP's definition of "merge" which is found at WP:MERGE. Right. IF you had read what WP means by "merge" you might have framed your proposal in some other terminology. BUT you didn't, and this has caused much confusion. SBHarris19:05, 7 April 2012 (UTC)
The template says "this article or section", my proposal listed some possible sections to merge. Merge proposal was good and some improvements have been made. GDallimore (Talk) 21:33, 9 April 2012 (UTC)
Oppose. The two articles can continue coexist; one with an emphasis on the visible aspect, the other with broader coverage. Binksternet (talk) 04:18, 7 April 2012 (UTC)
Do not merge I agree that this article should focus on visible light but of course it should mention that the term is sometimes used to refer to all EM radiation and give a link to that article, as it does now. Unnecessary duplication of material should be reduced, again by linking to specific articles. This may make this article shorter.
As a suggestion, perhaps there is then room to expand this article to include other literal aspects of visible light from the technical aspects covered here, artistic concepts of light, for example. Metaphorical uses of the word light (as in 'He has seen the light' etc) should not be covered here. Martin Hogbin (talk) 11:42, 7 April 2012 (UTC)
I oppose removing the history or special relativity section here, but support adding similar sections to electromagnetic radiation. The history section as it is currently written here is a nicely-written concise history of visible light, and therefore does not need to be substantially changed or shortened. Some redundancy between different articles (such as the history of the photon concept being discussed in "light" and also "EM radiation" and also "photon") is common and useful and appropriate, when it involves concisely-written sections--see Wikipedia summary style. --Steve (talk) 12:43, 7 April 2012 (UTC)
Not a great deal, which is why that section should be very short. The speed of light (another section) has more to do with visible light, since for the first 300 years of measurements of that speed, only visible EMR was used. Microwave speed of light testing didn't come until after WW II. I'll see what I can do about moving some of it. This can be done per discussion here and WP:BRD. No template needed. SBHarris19:10, 7 April 2012 (UTC)
I have to say that special relativity would, in my opinion, be a good thing to remove from this article. The article should concentrate on those aspects of visible light that distinguish it from the rest of EM radiation. Martin Hogbin (talk) 22:12, 7 April 2012 (UTC)
Can I close this proposal as not being done, except to move some stuff from visible light to this article, which I've done? I'm planning to put a few lines in, for each spectral class of EMR (as is done for radio and visible now), discussing how they are generated and when seen (their effects are discussed elsewhere). Then the bones of this article are pretty much laid down. What's left at light is far more specific to vis light. SBHarris21:08, 9 April 2012 (UTC)
Okay, I'm going to remove the merge label, since we've transferred a lot of material which was generic to EMR, to EMR, which was the intent of the proposer. Let me know, or post here, if there's anything else. SBHarris01:22, 19 April 2012 (UTC)
Poisson's interactions with Fresnel?
I was just looking at the page for Simeon Denis Poisson, where it says that he was a strong opponent of the wave theory of light. In the section wave theory of light, however, the article says that he wrote something in support of Fresnel's theory. I do not know about this subject, so which is it? I don't read French, so the citation on the Poisson page doesn't help.
Filodendron (talk) 21:39, 13 December 2011 (UTC)
HI my name is bob
] — Preceding unsigned comment added by 74.72.166.198 (talk) 18:49, 22 February 2012 (UTC)
While wave-particle duality is mentioned in the introduction, the sections which describe theories of light as particles and waves, seem to imply that the question has been resolved and that light is in fact a wave and does not behave like a particle. Shouldn't there be a reference to duality in that section?
Prov 1five (talk) 15:36, 19 April 2012 (UTC)
Yes, we should remind the reader in both cases that light is neither particle (like a baseball) nor wave (like a water wave), but has mathematical characteristics of both macro-things, depending on the circumstances. That fact that a thing can be described with baseball-like math or ocean wave-like math, does not mean it is a wave or a blob in the same sense that those things are. It's something we can't describe except by metaphor, because nothing behaves that way in our world. SBHarris00:27, 24 April 2012 (UTC)
Quantum Theory
There is a mistake under quantum theory that I can't edit. Currently it says:
"These light particles were named photons in In 1923 Arthur Holly Compton showed that the wavelength shift seen when low intensity X-rays scattered from electrons (so called Compton scattering) could be explained by a particle-theory of X-rays but not a wave theory. In 1926 Gilbert N. Lewis named these liqht quanta photons."
The beginning is a partial sentence that should be removed. The corrected version is:
"In 1923 Arthur Holly Compton showed that the wavelength shift seen when low intensity X-rays scattered from electrons (so called Compton scattering) could be explained by a particle-theory of X-rays but not a wave theory. In 1926 Gilbert N. Lewis named these liqht quanta photons." — Preceding unsigned comment added by Fortibus (talk • contribs) 17:41, 18 June 2012 (UTC)
Fixed. After 10 edits and 4 days, you'll be able to make such changes yourself. Welcome to Wikipedia. remember to sign your posts with four tildes: ~~~~. SBHarris17:47, 18 June 2012 (UTC)
Chronological Order
I can't edit right now, but the section on theories of light, which claims to be in chronological order, mentions the quantum theory before the theory of Maxwell. The two subsections should be switched in order. — Preceding unsigned comment added by 18.111.104.44 (talk) 04:31, 31 July 2012 (UTC)
Unit problem ?
I am a bit surprised by the dimension of lumen (luminous power). Shouldn't it be Watts ?
This should match the fact that candela is 1/683W per steradian of a specific source. SR has no unit => dimension is Watt for cd too.
Moreover it cannot be Joule. Joule is energy, if you wait, you have more. This would not be a characteristic of the led you buy. Regards. — Preceding unsigned comment added by Gonzalj (talk • contribs)
You're a bit confused. Part of the confusion is understandable, but part shows a lack of understanding. I'll try to explain, but this isn't really my area so I may not be of much help. If you don't understand what I'm saying, just accept you've got something mixed up and do some research of your own to try to learn what! :)
Most importantly, the candela is a base quantity like length and time. It cannot be expressed in terms of other units, unlike joules or watts which are derived quantities.
You're also confusing radiant intensity used in the definition of a lumen with luminous intensity, the actual quantity being defined.
Finally, you're (understandably!) confused by the symbol "J", which does not represent "Joules", but is here the dimension symbol for luminous intensity like L is the dimension symbol for length and T is the symbol for the time dimension.
Gonzalj -- Have you tried reading lumen (unit)? If so, I suggest you complain at Talk:Lumen (unit). Personally, I vote against giving any sort of definition of lumen or candela in this article...the advantage of having a self-contained article is (I think in this specific case) outweighed by the disadvantage of straying off topic and losing focus.
The article says that joule is the unit of radiant energy, watt is the unit of radiant flux. This is correct. Normally radiant flux is what people care about (how much light energy is being produced right now). An example where you might care about radiant energy is, maybe a skin cancer researcher would claim "If I have X joules of UV-B radiant energy over the course of my lifetime, my probability of skin cancer increases by Y percent." --Steve (talk) 01:09, 10 September 2012 (UTC)
The 'Rays' Of Light: As a Solid Proof Of 'Wave' Nature Of Light
The fact that we see the 'rays' of light, is a solid proof of 'wave-nature of light'. If light were like the spray of photons, then we should see a spray-like distribution of photons, and not the 'rays'. The waves travel in space making concentric-circles around the point-source-of-light. And interference of amplitudes of such circular-waves from many closely-located-point-sources give rise to rays of light.
Thus, the very observation of 'rays of light' is a proof of 'wave nature of light'. Hasmukh K. Tank — Preceding unsigned comment added by 219.91.215.243 (talk) 10:26, 6 February 2013 (UTC)
edit request
Whoever has editing privileges over this page: please correct, under the section "radiation pressure," the ungrammatical sentence beginning "The possibility to make solar sails..." to "The possibility of making...", and delete this request when finished. — Preceding unsigned comment added by 134.29.178.146 (talk) 17:28, 23 February 2013 (UTC)
Original source to Young and the Fig. 267 about water waves
In Youngs original work the description for the picture File:Young Diffraction.png is the next:
Fig. 267. Two equal series of waves, diverging from t A and B, and crossing each other in such a manner, that in the lines tending towards C,D,E, and F, they counteract each other's'effects, and the water remains nearly smooth, while in the intermediate spaces it is agitated...
In the chapter "ON THE NATURE OF LIGHT AND COLOURS", p.465 he says:
Supposing the light of any given colour to consist of undulations, of a given breadth, or of a given frequency, it follows that these undulations must be liable to those effects which we have already examined in the case of the waves of water, and the pulses of sound. It has been shown that two equal series of waves, proceeding from centres near each other, may be seen to destroy each others effects at certain points, and other points at to redouble them; and the beating of two sounds has been explained from a similar interference. We are now to apply the same principles to the alternate union and extinction of colours. (Plate XX. Fig. 267.)
Couls somebody correct the titel under the sketch "Thomas Young's sketch of the two-slit experiment showing the diffraction of light. Young's experiments supported the theory that light consists of waves." in the right citation "... Showing the diffraction of water waves."
"Above the range of visible light, ultraviolet light becomes invisible to humans, mostly because it is absorbed by the cornea below 360 nanometers and the internal lens below 400. Furthermore, the rods and cones located in the retina of the human eye cannot detect the very short (below 360 nm.) ultraviolet wavelengths, and are in fact damaged by ultraviolet."
This is unsourced and just plain wrong. The CIE table for photopic (color) vision range from 360nm to 830nm. Furthermore, CIE is not even concerned with the extreme limits of human vision, but with what ordinary people can see under ordinary circumstances. In the presence of exotic light sources such as lasers, diodes, filtered lamps, etc., or in vision experiments conducted by research ophthalmologists, people can perceive color and and have some visual acuity from about 310nm to 1064nm.
As for what the rods and cones can "detect," our own article on x-rays (subsection Visibility) indicates that they can detect x-days. Vision studies on aphakics (people missing a lens in at least one eye) can see down to 250nm with scotopic (rod-mediated) vision. As for the opacity of lens and cornea, that's not an all-or-nothing proposition. People can see 10-13 orders of magnitude of brightness, 5 at one time. If the opacity of lens goes up by a few orders of magnitude, the effect is no different than putting on a dark pair of sunglasses: turn up the light and you can see just fine.
The part about "damaged by ultraviolet" is irrelevant. Being outdoors in the daytime without broad-brimmed hat, long-sleeve shirt, and uv-blocking glasses is bad for your health, but has nothing to do with whether you can see. Zyxwv99 (talk) 21:49, 23 September 2013 (UTC)
Agree with all you write. Can you find some cites for us so we can put it in? The uncited statement in the article now unfortunately is the sort of thing that "everybody knows but just isn't so ". So if we get into a cite war we 'll get a lot of poor quality tertiary sources that are dealing with this only in passing and repeating common wisdom. SBHarris16:44, 24 September 2013 (UTC)
My notes are a little disorganized, but here's what I have so far:
"In young adults, wavelengths as high as 1000 nm or down to 300 nm may be seen, but the standard range for human vision is typically given as 400-700 nm."
Scott E. Umbaugh
Digital Image Processing and Analysis: Human and Computer Vision Applications with CVIPtools
http://books.google.com/books?id=UQTMw5uoGHgC&pg=PA405
"Limits of the eye's overall range of sensitivity extends from about 310 to 1050 nanometers, but strong illumination is necessary for sensation at these wavelength extremes."
David K. Lynch, William Charles Livingston
Cambridge University Press, 2001
Color and Light in Nature
http://books.google.com/books?id=4Abp5FdhskAC&pg=PA231
"According to different authors, under appropriate conditions seeing is possible in the ultra-violet down to a wave-length as small as 3100 Å. This fact has been confirmed on 21 persons (age 25–50 years)..."
W. de GROOT
Seeing in the Ultra-Violet
Nature 134, 494-494 (29 September 1934) | doi:10.1038/134494a0
http://www.nature.com/nature/journal/v134/n3387/abs/134494a0.html
"The foveal sensitivity to several near-infrared laser wavelengths was measured. It was found that the eye could respond to radiation at wavelengths at least as far as 1064 nm. A continuous 1064 nm laser source appeared red, but a 1060 nm pulsed laser source appeared green, which suggests the presence of second harmonic generation in the retina."
The opening sentence: "Visible light (commonly referred to simply as light) is electromagnetic radiation that is visible to the human eye,& people sitting in toilets and is responsible for the sense of sight" gives the impression that light is visible. However the standard scienfic view of light to the best of my understanding is that only the manifestations of light are visible and not light itself. Though I cannot immediately think of a best sentence, I propose that this issue may be addressed in some way. --Nagarjunag (talk) 12:09, 25 October 2013 (UTC)
Light,opening description needs context for the layman.
Light whether visible or invisible is electromagnetic radiation, that occupies a small narrow part of the electromagnetic spectrum, between 310nm and 1050nm.
Visible light is normally between 400nm and 700nm.
Light, emitted or reflected carry's image information to the eye, in the form of photons and waves.
Lights photons are small quantifiable packets of energy, that could be imagined as tiny glowing white particles, that move in straight lines, longitudinally.
Lights waves are reactionary undulatory movements in the semi-solid medium of space that surrounds the photon, that could be imagined to be like bow waves around a boat, that move sideways transversely, and despite their invisibility account for all the colours, when illuminated by background photons moving over them, i.e. due to the inertial quality of the photon, the photons frequency is governed by the size of the transverse wave it passes over, i.e. if the transverse wave is 450nm in height, the photon will move up and down 450nm and continue to oscillate at 450nm for the rest of it's journey to the eye, creating a blue colour.
A Newtonian telescope reveals a rainbow edge around objects, which is composed of transverse waves, illuminated by background photons.
Transverse waves can be seen in this photo, moving through a glass prism, they look like ripples moving through water, which exit the prism creating rainbow colours when illuminated by photons.
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Several citations needed, as claims are made without having a valid source:
First citation is needed for:
The speed of light in a vacuum is defined to be exactly 299,792,458 m/s (approximately 186,282 miles per second). The fixed value of the speed of light in SI units results from the fact that the metre is now defined in terms of the speed of light. All forms of electromagnetic radiation move at exactly this same speed in vacuum.
Second citation is needed for:
Léon Foucault used an experiment which used rotating mirrors to obtain a value of 298,000,000 m/s in 1862. Albert A. Michelson conducted experiments on the speed of light from 1877 until his death in 1931. He refined Foucault's methods in 1926 using improved rotating mirrors to measure the time it took light to make a round trip from Mt. Wilson to Mt. San Antonio in California. The precise measurements yielded a speed of 299,796,000 m/s.
Third citation is needed for:
The effective velocity of light in various transparent substances containing ordinary matter, is less than in vacuum. For example the speed of light in water is about 3/4 of that in vacuum. However, the slowing process in matter is thought to result not from actual slowing of particles of light, but rather from their absorption and re-emission from charged particles in matter.
About William Herschel experiments who discovered the infrared I heard the colors (wavelenght - may references to "color temperature" ) have different temperature effect. Is this dont need to be mentioned? --Beyond silence07:11, 25 February 2014 (UTC)
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Under the heading "Speed of Light", the text reads "The precise measurements yielded a speed of 299,796,000 m/s".
Michelson's final paper assisted by F. Pearson states the speed of light found as 299,796,000 m/s. His paper can be found at this link and so should be added as a source.
[12]Tom.Matthewson (talk) 02:01, 12 March 2014 (UTC)
Not done: as you have not requested a change. If you want to suggest a change, please request this in the form "Please replace XXX with YYY" or "Please add ZZZ between PPP and QQQ". Please also cite reliable sources to back up your request, without which no information should be added to any article. - Arjayay (talk) 16:42, 18 August 2014 (UTC)
Error under "Quantum Theory" section : reference to baseballs as a macroscopic phenomena
Go to Light section "Quantum theory" & read 2nd paragraph. See word "baseballs" at end which is either an absurd joke or makes no sense:
"Eventually the modern theory of quantum mechanics came to picture light as (in some sense) both a particle and a wave, and (in another sense), as a phenomenon which is neither a particle nor a wave (which actually are macroscopic phenomena, such as baseballs or ocean waves). "
Baseballs are a macroscopic phenomena? Really? Please correct this error.
I just read the offending section. It looks fine to me. "Macroscopic" means big enough to be seen with the naked eye. The paragraph is explaining that our conceptions of waves and particles were invented to deal with things large enough to be seen without a microscope, like baseballs and ocean waves, but that these concepts don't seem to adequately describe things at the quantum level. Instead, waves and particles can describe some aspects of what's going on. Zyxwv99 (talk) 14:22, 10 January 2014 (UTC)
I agree with this interpretation. The television series The Universe had actually performed a demonstration of the wave/particle duality of light using baseballs; it is a fairly common analogy and makes sense in this context.
Re Bent straw refraction illustration. Please consider changing the illustration or the text so that the physical bend near the top of the straw will not be confused with the refraction that you are illustrating near the bottom of the straw. I freely acknowledge that no thoughtful adult would be confused. Nevertheless, it would remove potential source confusion to have only one bend or to explicitly indicate the area of interest. Harry1948 (talk) 08:19, 11 November 2014 (UTC)
I'm not sure I fully understand this request. Both bends are shown to illustrate that the speed of light has changed upon entering the new medium (water) and changed back upon exiting and into its original medium (air). How exactly would you suggest this be depicted as to avoid confusion?
He's talking about the fact that the straw itself also has its own bend to facilitate drinking -- as opposed to the apparent bend as it enters the water. I think it's a very poor illustration anyway because the refractive bend is barely noticeable through the top surface of the water while the displacement from the simultaneous view through the side of the glass is much more obvious. To illustrate just the apparent bend from refraction we would need a photo that only shows the view through the top surface of the water. DavRosen (talk) 20:13, 21 November 2014 (UTC)
Assessment comment
The comment(s) below were originally left at Talk:Light/Comments, and are posted here for posterity. Following several discussions in past years, these subpages are now deprecated. The comments may be irrelevant or outdated; if so, please feel free to remove this section.
Please note the following:
"The speed of light in a vacuum is exactly 299,792,458 meters per second (fixed by definition). Although some people speak of the "velocity of light", the word velocity is usually reserved for vector quantities, which have a direction."
Last edited at 10:18, 6 May 2007 (UTC).
Substituted at 20:42, 3 May 2016 (UTC)
Done--Oneiros (talk) 19:27, 27 January 2010 (UTC)
Not done: That's right. Four days and ten edits will make you autoconfirmed. You are at eleven days and nine edits. Just make one more edit and you will be able to edit semi-protected pages. Welcome, Celestra (talk) 15:13, 7 February 2012 (UTC)
Done - Arjayay (talk) 18:20, 17 February 2014 (UTC)
Not done: as you have not requested a change.