Wikipedia:Reference desk/Archives/Science/2007 December 27

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

December 27

Triple vision

I'm nearsighted. When I look at a light-to-dark transition (say, a dark door against a white wall) with my glasses off, I don't see a smooth transition, but rather two or three distinct edges. Why is this? --67.185.172.158 (talk) 02:08, 27 December 2007 (UTC)

Two distinct edges is easily explained if your two eyes are failing to fuse a stereo image into a single image - or if (for example) one eye is focussing the image differently from the other. Three distinct images would be very tough to explain though...are you SURE you ever see three distinct images? SteveBaker (talk) 07:10, 27 December 2007 (UTC)

As you would know, unfocused vision causes what should be a dot to spread out into a circle of confusion. This has a definite size that depends on the amount of the focusing defect. If you look at a sharp black/white boundary, you will see the white penetrate into the black by an amount equal to the radius of the circle of confusion, forming a dark gray stripe; and (less obviously) the black penetrates into the white by the same amount, forming a light gray stripe. But you can also perceive the boundary in its true position to some extent (as a boundary between the light and dark gray), because some part of the misfocused light is offset parallel to the boundary or nearly so, and doesn't impair your detection of it. This can create the effect of a triple image. I'm myopic and in some circumstances I can see this type of triple image with one eye closed, although sometimes not all three boundaries are visible, so then I get a double image. In addition, if your eyes are myopic to different degrees, the circles of confusion for the same object will be different sizes, which may also cause the appearance of additional images of the boundary: logically there could be as many as five, although I have not seen this. --Anonymous, 10:15 UTC, December 27, 2007.

A Mach band

Ah - I see what you're getting at. I don't like your explanation though. If you can see this effect with one eye alone then I think you are seeing a phenomenon called Mach banding (see image at right - sadly, our article on the phenomenon is rather weak). The deal is that if you plotted a graph of brightness versus X coordinate of this defocussed edge, the real world looks like this:

                 ____White
                |
  Black_________|

but with somewhat blurry vision, it would look something like:

                 ____White
                /
  Black________/

The human brain is very attuned to the nature of colour brightness transitions and tends to interpret shading as shape. In order to do that, discontinuities in the derivative of the light function are accentuated in our minds to make the changes look more prominant. That's the cause of Mach banding. Look at the image at right which has a brightness-versus-X curve like this:

  Light Grey____
                \
                 \_______Dark grey

But it looks to us mere humans like this:

  Light Grey__|\
                \
                 \  _____Dark grey
                  \|

So you see a brighter line just before the colour transition and a darker line just afterwards. This would be a mechanism that would produce these 'double edges' with just one eye.

It's also possible for a particular kind of optical defocus to cause a third discontinuity in the derivative in the middle of the ramp between dark and light that would produce a third 'edge'.

SteveBaker (talk) 17:59, 27 December 2007 (UTC)

I don't like your not liking my explanation, because I think we're both right. I'm talking about where the transition comes from and you're explaining why it might be perceived as a sharper edge than otherwise. --Anon, 21:35 UTC, December 27.

Also a door might actually have three or more transitions because it has a wooden frame in a wall. – b_jonas 09:03, 28 December 2007 (UTC)

grapes in a microwave

As seen in this video. Why do grapes do this? What's actually going on when the grapes are put inside the glass? 70.162.25.53 (talk) 06:17, 27 December 2007 (UTC)

I found something among the comments to the video. The fifth page of the comments, post by one ' thelaughingjackle'. Awesome video by the way. --Ouro (blah blah) 11:28, 27 December 2007 (UTC)

In case anyone doesn't want to page all the way to "thelaugingjackle"'s response, here's the process that was described (I'm paraphrasiong what was said to avoid possible copyright issues):
1. The two halves of the grape become charged with negative and positive electromagnetic poles as soon as the microwave is turned on.
2. Immediately, the water contained in the grape starts to go through a process of electrolysis, which breaks the water molecule down into oxygen and hydrogen atoms, which then accumulate at either magnetic pole.
3. As soon as hydrogen gas is relased, A tiny electrical spark is created by the grape being excited by the microwaves. This spark then ignites both the hydrogen and oxygen gases being released, producing a plasma fire above the grape.
4. The hydrogen gas burns as an orange red, and the oxygen gas burns as a blue.
One thing for sure, that is indeed a very cool video! -- Saukkomies 10:34, 27 December 2007 (UTC)

Hmm, you reckon??? that 'laughingjackle' must still be laughing. Richard Avery (talk) 15:54, 27 December 2007 (UTC)

I find step one rather tricky. why doesn't this happen when I put two potatoes or a bowl of broccoli florets in the microwave? William Avery (talk) 21:48, 27 December 2007 (UTC)

My unresearched guess would be that it was to do with a larger percentage of water in grapes and less porous skin. More water means it absorbs more energy from the microwaves. A less porous skin means that the electrolyzed gases can't simply pass through the skin, creating a build up of flammable gases. Fructose being easier to burn than complex starches may or may not also be a factor. EvilCouch (talk) 04:33, 28 December 2007 (UTC)

There may be some significance in the way the grape was cut, to have two ends whose connection had a relatively tiny cross-section. Someguy1221 (talk) 05:03, 28 December 2007 (UTC)

Looked like a plasma in the glass.--TreeSmiler (talk) 05:20, 28 December 2007 (UTC)

This [1] may indicate what is happening.--TreeSmiler (talk) 05:36, 28 December 2007 (UTC)

Doesn't explain why it works for grapes and not other stuff though. If it just relies on electrolysis of water to produce hydrogen and oxygen then why can you not do it with a bowl of saline solution? (At least I don't think you can! I know for sure that heating up drinking chocolate or coffee doesn't). I go with Someguy1221 that the cutting of the grape is significant. An electric field will develop between the two pieces. Because the join between them is narrow it is also high electrical resistance and will result in most of the ohmic heating from the current flow heating up that small region. Quite how that ends up producing a plasma I'm not sure.

Spinningspark (talk) 14:02, 1 January 2008 (UTC)

My own belief (not sure where I heard this) is that the two grape halves form a dipole antenna well-tuned to the wavelength of microwaves.

A little while back some friends of mine went to try it, but didn't have any grapes handy. We discovered that nice, plump blueberries worked just about as well. —Steve Summit (talk) 21:51, 1 January 2008 (UTC)

Hmm. Looks like I'm totally wrong about the antenna principle. According to my calculations and our Microwave oven article, the wavelength of microwave oven microwaves is about 12 cm, hardly grape-sized. —Steve Summit (talk) 22:07, 1 January 2008 (UTC)

Actually your antenna idea is correct. No water is required, and this has nothing to do with electrolysis or hydrogen. Don't forget that antennas need not be resonant to quarter- or half-wavelength, and can be any size (e.g. the ferrite antenna inside a portable AM radio.)

The "grape blowtorch" effect is mostly based on the fact that, in a nearly-empty microwave oven, the e-field strength rises to nearly the breakdown threshold for air. This strong e-field induces a fairly large current in any conductive object oriented in the same direction as the e-field. The grape acts as an "antenna," and the maximum current density is at the joint between the two halves. This thin section boils, drys, and carbonizes, then breaks in half, creating an electric arc full of carbon ions. The small region of arc-plasma between the grape-halves is then inductively heated by the strong microwave e-field. It grows larger and less dense, and begins to rise. Once parted from the conductive grape halves, it continues to absorb energy from the e-field, becoming an "electrode-less discharge" or "plasmoid." Sometimes these plasmoids will crawl around on the ceiling of the microwave cavity for as long as the power remains on. They'll burn off the paint though. The loud buzzing sound is caused by the 120Hz ripple on the microwave generator's power supply, and is an example of a "plasma loudspeaker."

A similar phenomenon occurs with most any source of corona discharge in the oven. Sharp edges of foil, burrs on metal cans, or sharp utensils sometimes launch similar "plasmoids," though not as reliably as the grape-halves. The main requirement is that the oven cavity lack a large electrical load in the form of food or water. Without a load, the cavity "Q factor" is very large, leading to resonant step-up and high voltage effects. The second requirement is for carbon or sodium ions in the arc. If contaminated by metal salts, the plasmoid puts out very bright light of characteristic color. (ANd another trick is to prop up the carbon "lead" from a mechanical pencil, and watch it launch huge blasts of plasma.) These plasmoids are basically the same as "Saint Elmo's Fire," but since they couple capacitively/inductively to the high frequency AC source, they can persist even when not attached to any conductor. --Wjbeaty (talk) 23:20, 2 January 2008 (UTC)

Wouldn't it be easy to test (with a magnet) if it was really a form of plasma or just some sort of heated gas? --24.147.86.187 (talk) 18:53, 2 January 2008 (UTC)

You could still have something there Steve. Don't forget, the elements of a dipole (like most antenna) are λ/4, not λ in length. So 3 cm is still a bit big for grape sized, but not so far off and the base of the grape may be forming a dielectric substrate reducing the wavelength still further. Spinningspark (talk) 19:21, 2 January 2008 (UTC)

Tree hollow vs tree hole

What's the difference between a tree hollow and a tree hole —Pengo 06:30, 27 December 2007 (UTC)

Looks the same to me. Guettarda (talk) 06:38, 27 December 2007 (UTC)

Hollows are formed "naturally" holes are formed by breakages of branches. Hollows are "semi enclosed" —Preceding unsigned comment added by Shniken1 (talk • contribs) 06:57, 27 December 2007 (UTC)

Those descriptions fit both. I think they're the same. —Pengo 23:46, 27 December 2007 (UTC)

I've given up looking for differences and merged the two articles. —Pengo 00:24, 28 December 2007 (UTC)

Morphine Effects

How does Morphine achieve its pain killing effects. All sources I cheacked say that it does this by blocking the opioid receptors. But they never mention who that helps. Does the blockage obstructs pain signals to the brain, or allows the brain to ignore it or what? is there any good links I can read up on? —Preceding unsigned comment added by 99.240.203.201 (talk) 14:28, 27 December 2007 (UTC)

The short answer to your question is that the Morphine does block the pain signal to the brain. It does this by plugging up opioid receptors between the nerve cells (called Neurons) in the body, so that signals from a part of the body that indicate pain is occurring are not passed on from one nerve cell to the next - and thus no signal reaches the brain. The way these signals are passed between nerve cells is based on chemistry - there are specific chemicals called neurotransmitters that are in the tip of each nerve cells' little branches, called Axons - and which travel the short distance from one nerve cell to the next when excited to do so (when the cell gets a signal to pass on to the brain). The special message centers located on the end of the nerve cell's branches are called Synapses. When a synapse receives a message to pass on to the next nerve cell's synapse, it releases a flood of neurotransmitters which travel the short distance between the two nerve cells. There are specific receptors to which the neurotransmitters need to go in order for the receiving cell to know to pass the signal on up the chain to the brain. Different receptors are chemically constructed to allow only specific chemicals to fit - sort of like having the precise key to fit into the precise lock. If the receptors are all clogged up with another chemical that is similar to the neurotransmitter being sent by the sending nerve cell, then there's no way for the neurotransmitter chemical to attach itself to the receiving nerve cell's surface - and this blocks the message from being passed on from one nerve cell to the next. Morphine happens to have a similar chemical structure to the "Mu" neurotrasmitter that passes on many of the pain signals that the body sends to the brain. So when someone has morphine introduced to their body, it spreads around and plugs up these nerve receptors, and this blocks the pain signals from reaching the brain. The reason morphine, and all other opiates, are so highly 'tolerant" is because the body compensates for all these plugged receptors by creating more of them on the ends of the nerve cells. This in turn means that a person needs to take more and more of the opiate (morphine) to achieve the same results - it's the body's way to overcome the need for the brain to receive pain messages. -- Saukkomies 11:05, 27 December 2007 (UTC)

does Morphine also inhibit the prodection of substance P? —Preceding unsigned comment added by 99.240.203.201 (talk) 16:26, 27 December 2007 (UTC)

Couples

Given a number of forces acting on an object, how can one find the equivalent couple? The article couple (mechanics) doesn't help, neither do any of my textbooks. Laïka 14:30, 27 December 2007 (UTC)

Well, the simplest way is to calculate the net torque from the forces you have. The equation for that is the second one in the article:

$\tau =F\times r_{0}+-F\times r_{1}=F\times (r_{0}-r_{1})=Fd{\hat {e}}$

Then you can use the first equation: $\tau =F\times d\,$ (torque = force x distance - which applies when there are two equal and opposite forces, each acting at some distance that are acting as a couple). Since there are an infinite number of possible force/distance values that satisfy your criteria, either pick a fixed distance and use the equation to calculate the force - or pick a force and calculate the distance.

SteveBaker (talk) 17:30, 27 December 2007 (UTC)

Ok, so when my question says "the equivalent couple" (with no other qualifying info), it really just means "an equivalent couple". Thanks, Laïka 20:00, 27 December 2007 (UTC)

Yes. SteveBaker (talk) 23:33, 27 December 2007 (UTC)

My understanding is that the couple is measured in units of torque, so there is a unique value (distance X force). If you so choose to decompose this into a force and a distance, then as SteveBaker has mentioned, selecting either will define the other. Nimur (talk) 01:54, 28 December 2007 (UTC)

What is amitotic?

My question for the reference desk is:

In your article on "Glial cells" I found the word "amitotic". When I searched for amitotic I got the message No page with that title exists. I also searched Wiktionary and got the same results.

What is amitotic?

Billtobill (talk) 15:25, 27 December 2007 (UTC)Billtobill

amitotic = not engaging in mitosis

"Direct cell division by simple cleavage of the nucleus, without spindle formation or the appearance of chromosomes." [2]

Dragons flight (talk) 15:33, 27 December 2007 (UTC)

So how is the DNA/chromosomes distributed among the daughter cells? --NorwegianBlue ^talk 15:52, 27 December 2007 (UTC)

I've changed amitotic to post-mitotic. See ref: doi:10.1038/nrn2124. The idea is that they've stopped dividing at some point in human development, not that they never could. Furmanj (talk) 16:19, 27 December 2007 (UTC)

The linked abstract was about neurons, not about glial cells, so I'm not sure that the change was correct. Sorry, didn't read our article properly, and thought amitotic was supposed to refer to glia cels, which made no sense. The change was justified. --NorwegianBlue ^talk 17:47, 27 December 2007 (UTC)

Free basic chemistry textbook?

I'm looking for a free, online basic chemistry textbook. I know about wikibooks, but the General chemistry book is quite a bit from being finished. Any advice? It should start out simple with describing atoms, protons, electrons etc, and move on to the periodic table, bonds, reactions etc. Thanks in advance! Aeluwas (talk) 16:30, 27 December 2007 (UTC)

Hey Aeluwas, I just finished taking chemistry, these sites may be of interest to you: [3] and [4]. If you really want to learn chemistry, use your book, practice, and don't be afraid to ask a teacher or [teaching assistant] --n1yaNt 00:30, 28 December 2007 (UTC)

For high quality, free and legal courseware, I can recommend the MIT site [5]. There you will find a myriad of science materials (1800 courses at present).Mr.K. (talk) 02:33, 28 December 2007 (UTC)

Cleaning elderly and disabled people

Are there any machine to clean elderly and disable people? What is the 'cleanest' way of cleaning an adult? (pun not intended). Mr.K. (talk) 19:24, 27 December 2007 (UTC)

Yes. I don't have an answer to the second question, though. Someguy1221 (talk) 20:25, 27 December 2007 (UTC)

I just knew it was going to be a Japanese invention before clicking on the link. They appear to be on the forefront of robotics and related areas particularly in relation to the elderly. As for the second question, what do you mean 'cleanest' way? You could soak them in some kind of acid but you'd end up killing the person as well. Nil Einne (talk) 13:16, 28 December 2007 (UTC)

The Japanese are furthest ahead in this regard because they have a shrinking population coupled with a sharp increase in life-spans. They realise that in time, care of the elderly will become very difficult because of a shortage of younger people to look after them. The drive to find robotic assistants for the elderly is therefore a significant stimulus for their industry to carry out this kind of research. SteveBaker (talk) 14:51, 28 December 2007 (UTC)

This is where we invest our declining US dollars. Robotic elder care. I'm not kidding! Saudade7 20:37, 28 December 2007 (UTC)