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April 30

Photo showing fluorescence in calcite

Yesterday on the Main Page there was this photograph of a calcite crystal, with the caption indicating that it shows fluorescence and birefringence. The birefringence is obvious but I was puzzled about the fluorescence and nobody responded to my comment on the talk page of the Main Page requesting an explanation. Fluorescence normally is observed as a change of frequency between the source and the emitted light, whereas here the light leaving the crystal seems to match the colour of the light going in. Within the crystal the light looks yellower, but I am not enough of a physicist to know whether this is the effect that demonstrates fluorescence. I can envisage that the absorbed then re-emitted light is given off omnidirectionally, and that this traces in yellow the path of the light rays that we see within the crystal. But why does the strong blue of the source disappear within the crystal and then reappear outside it again? Although the filename of the photo is Fluorescence_in_calcite.jpg, I note that the Calcite article only uses it as an example of birefringence. Thanks. Jmchutchinson (talk) 07:32, 30 April 2022 (UTC)[reply]

The blue beam is only visible when the blue light is scattered, for example, by dust particles. Due to the 1.3s exposure time, no individual dust particles can be seen. Dust particles are present in the air, but not in the crystal. In the crystal some part of the blue light is absorbed and re-emitted as orange light, which is called fluorescence. Most of the blue light just continues to the other end of the crystal.

On the lower left you can see the laser beam going in. Part of the light is reflected on the front surface of the crystal; this is the beam you see on the upper left. In the crystal, the two polarisations are refracted by different angles, forming two beams in the crystal. At the end surface, the two beams (both of which are polarised along the axes of the crystal) are refracted back to their original direction, forming the two parallel beams on the right. Inside the crystal, the two beams cause fluorescence and non-polarised orange light is emitted from each point along both beams in every direction. As this light leaves the crystal, it's separated into two orthogonal polarisations, which are refracted by different angles, so that the observer sees four orange beams. The additional spots and beams you can see result from internal reflections. PiusImpavidus (talk) 10:35, 30 April 2022 (UTC)[reply]

That's a very clear explanation: thanks! I think it will be good to add it to the Commons file for this image (currently still protected). Jmchutchinson (talk) 11:18, 30 April 2022 (UTC)[reply]

Fantastic response! —Amble (talk) 16:30, 1 May 2022 (UTC)[reply]

Thanks. I copied my description to the image caption on commons. PiusImpavidus (talk) 07:35, 2 May 2022 (UTC)[reply]

A US engineer question

How can a buildings project manager, working under non-PEs, satisfy the design work experience requirement for the Professional Engineer license? If you only work with consultants, and other stakeholders...and who will sign the forms anyway? Thank you.+ Imagine Reason (talk) 13:00, 30 April 2022 (UTC)[reply]

It depends on the state. Talk to someone in the engineering society for your state. There may be a branch of the National Society of Professional Engineers or of the society for your specialty, such as the American Society of Mechanical Engineers. You could try your state licensing board, but the people who answer the phones are sometimes government bureaucrats who don't actually understand engineering. Jc3s5h (talk) 13:29, 30 April 2022 (UTC)[reply]

This is not an uncommon situation. Contact your professional engineer association. They will probably require you to keep a diary, which will need to be signed by your work experience boss to establish that you actually did the work listed in your diary, and be then signed by a member of the professional engineer association nominated or recommended by the association to establish that your experience is relevant. The nominated member might wish to interview you to satisfy him or her that you have an understanding of the work. He/she may also guide you in actually getting the right sort of work - its not unknown for companies to give work experience chaps nothing but chores such as sweeping the floor, fetching tools, and the like. You need to tread carefully sometimes to be allowed to do actual engineering. Dionne Court (talk) Dionne Court (talk) 09:49, 4 May 2022 (UTC)[reply]

Sources about Peninsula

I'm looking for any useful sources (book, web, or otherwise) about peninsulas, for use in researching for Peninsula. I've found a small amount, but nothing substantial, in the form of web articles, and some are suspicious looking in terms of reliability. Also, there were a bunch of tertiaries. I found a few books on the OpenLibrary that were dictionaries of physical geography, but in each, I just found a one line definition. Any other search for books didn't end well. I found a primary school book which funnily enough seemed the most in-detail book I'd found so far, but it seems silly to use a source made for children (unsure about reliability), and I have no way to access the book other than buying it.

Any help in finding sources would be greatly appreciated! — PerfectSoundWhatever (t; c) 20:57, 30 April 2022 (UTC)[reply]

The Creation of Peninsulas. Alansplodge (talk) 08:58, 1 May 2022 (UTC)[reply]

Thank you — PerfectSoundWhatever (t; c) 14:49, 1 May 2022 (UTC)[reply]

I've since added 4 book sources to the article, but any more not on the list would still be amazing. Thanks! — PerfectSoundWhatever (t; c) 21:15, 2 May 2022 (UTC)[reply]

What kind of radio signals can i pick up with an old TV aerial?

Hello, sorry if this is the wrong sort of post for here, just like many houses there is an old VHF Yagi-type analog TV aerial on the roof, that terminates in a connector in my bedroom, what sort of signals other then the lack of TV could i rx if i plugged it into my receiver?

Sincerely OGWFP (talk) 21:28, 30 April 2022 (UTC)[reply]

You should be able to receive some FM radio. Also if you have a scanner, you can probably receive the airband VHF band, and some VHF mobile channels. But the antenna should point roughly at the transmitter! Graeme Bartlett (talk) 03:49, 1 May 2022 (UTC)[reply]

Thank you, as it's a TV aerial it's pointed towards a TV station and doesn't have a rotator so i don't think i'll bother.

Sincerely OGWFP (talk) 16:08, 1 May 2022 (UTC)[reply]

In the United States, outdoor TV antennas (the large ones with all the poles sticking out of it) pick up a range from about 50 MHz to 250 MHz. FM Radio is 88 to 108 MHz. So, you can use the old TV antenna as a good FM radio antenna. CB radio won't likely work. It is too low a frequency. If the antenna has UHF along with VHF, there is a new range to search. Also, if you get a European antenna, it likely has multiple other frequency ranges because they have multiple frequency bands depending on the area. 97.82.165.112 (talk) 14:16, 3 May 2022 (UTC)[reply]

Your tv antenna is likely to be arranged to pick up horizontally polaried signals (like most VHF/UHF TV). Most other signals are vertically polarized, including FM. FM signals were originally horizontally polarized but now are vertical or circular polarized (Antenna basics - Radio World).

If you turn the antenna on it's side it will be better at picking up vertically poloarized signals such as amatuer radio, public safety, CB, walkie-talkies, etc. depending on your receiver.

Check out socalscanner.com for what scanners can pick up. 74.92.173.225 (talk) 22:37, 3 May 2022 (UTC)[reply]

Many TV aerials are engineered to have a gap in sensitivity for the FM band. This helps performance on TV bands and reduces interference from nearby FM stations. In countries and locations that had few analogue TV stations (eg Australian cities which had only channels 2, 7, 9, and 10 in use), the TV aerials were engineered to work well on only those channels, reducing complexity and cost (and reducing interference from 2-way radios and military communications). Dionne Court (talk) 09:37, 4 May 2022 (UTC)[reply]