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I shone a green laser light(~532 nm) through a glass slab and what I saw inside was that the light beam was now red. Imagine a regular refraction diagram but color the beam outside green and that on the inside red.

As frequency doesn't change upon entering another medium, effective light velocity in a medium becomes directly proportional to that of wavelength. So, as speed decreases shouldn't wavelength also do so? But in this case, it increases. Or is this logic wrong?

Also, it can't be due to light losing energy.

What is happening in here?

Adam Karlson
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  • What color is it after it exits the back side of the glass? – garyp Apr 01 '16 at 16:11
  • But it's green when it comes out... And also it is not because the light is being seen through the slab... I kept the slab above the green beam, and looked from above- it was still green. Conclusion: it only occurs when beam PASSES THROUGH the slab. – Adam Karlson Apr 01 '16 at 16:11
  • What the eye sees outside the glass slab as green, is the scattered light on molecules of air. What the eye sees when the beam goes through the slab is the scattered light on molecules of glass. As you say that the beam looks again green coming out of the slab, the frequency of the scattered light does not change in the air, an elastic scatter. My first guess is that the scatter in the glass is inelastic and the red color appears ( which could be a perception of red by the retina https://en.wikipedia.org/wiki/Color_vision ). i am waiting for an optics expert – anna v Apr 01 '16 at 17:21
  • Um...Anna. I have also tried looking at the beam through a glass and I can see green. Shouldn't any part of that light going through the glass interact in the same way, thus producing red every time? It's a beam WHICH PASSES THROUGH THE SLAB WHICH IS REDDENED not one being looked at. I hope I am clear, (wish I could explain better) – Adam Karlson Apr 01 '16 at 18:47
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    Please describe the glass in detail -- what is its composition? Is it a standard optical glass, like BK7? Also what is the power of the laser beam? Some glasses can fluoresce, which could give your red color; if the laser fluence is high enough the glass may fluoresce. Also what type of laser goggles are you wearing; these can alter color perception. – Peter Diehr Apr 01 '16 at 21:21
  • @AdamKarlson The scattered light outside is elastic, through the glass the photons coming to your eye have not scattered from their original direction in the glass ( to change color) because the probability is very small. It is the huge number of photons in the laser beam that allows for observable scatter. – anna v Apr 02 '16 at 03:17
  • @Peter Material: Not sure, but it's standard laboratory glass slab. Power: It says <1000mW Nope, no goggles. – Adam Karlson Apr 02 '16 at 09:53
  • @Anna v... Ok, external scattering results in green, while inelastic internal scattering results in red. Hypothetically, What if i look at the green laser beam ( which one shouldn't), say the beam is 2 feet away. Now, I place a glass slab 1 feet away from myself and look at the beam through it? It's hardly visible but green! The low probability explains green, but why do not these photons change color now?Why? – Adam Karlson Apr 02 '16 at 10:07
  • use a piece of paper, or a film, for the beam to fall on. The bulk of photons is elastically scattered and keeps the frequency. I expect you should see a thin halo around the green spot that is red, from the few inelastically scattered inside the glass. ( because they have scattered there will he an angle) – anna v Apr 02 '16 at 10:25
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    @adam karlson: <1.000 mW? A green laser pointer? Or what? If a laser pointer see https://www.technologyreview.com/s/420214/the-danger-of-green-laser-pointers/ They often emit more than one wavelength. I can see 780 nm scatterred in air, so possibly you are seeing 808 nm leakage scattered in the glass. Cheap green laser pointers lack adequare filters, and should be tossed. Test with another, newer laser source. – Peter Diehr Apr 02 '16 at 11:13
  • @AdamKarlson just letting you know, writing in all caps is generally considered somewhat rude on the internet. Please avoid it in the future. – David Z Apr 04 '16 at 16:22
  • Yes, I know that, but it is akin to raising your voice, which is what I did just to emphasise. The reason why it's rude is because raising one's voice in public is considered rude. Sorry, for the Non-Physics comment – Adam Karlson Apr 04 '16 at 17:24
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    Having just seen this, I decided to try my luck using a 532 nm green laser pointer, 535 nm interference filter (to eliminate any 808 nm light the laser might also inadvertently emit) and a glass plate 1/4” thick and about 2.5” square. All room lightsout, I can see a faint green trace for the light passing through the glass plate. No red and way too dim for me to photograph. So, a null result, but I don’t doubt your observation. Personally, I think it is fluorescence from some impurity in the particular piece of glass you have (or had). Anyway, good question to upvote. Raman seems improbable. – Ed V Apr 18 '23 at 22:33
  • Related with photos: https://physics.stackexchange.com/q/772103/123208 – PM 2Ring Aug 18 '23 at 10:37

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I also noticed this happening, also with a 532nm green laser, but the beam inside was somewhat more orange than red. This only happened with one particular piece of glass that I had, the sphere from this set: https://www.amazon.com/dp/B08CZ9PCJP/?ref_=cm_wl_huc_item It is advertised as "k9 material", and other than this color change, I have no reason to believe it is a different material. it may be due to the shape, but changing the angle of incidence does not change the color. I tried to shine through different types of glass laying around my house, and water too, and I found one particular whiskey glass that does a similar (but far less noticeable) color change. After further reading, it seems to be raman scattering, a type of fluorescence, and is usually due to Ge impurities in the glass.

Garmysheva, T.Y., Shendrik, R.Y., Paklin, A.S. et al. Luminescence of Oxygen-Deficient Centers in Quartz Glasses. Glass Phys Chem 48, 232–235 (2022). https://doi.org/10.1134/S1087659622030038

Jason Killian
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    Are you sure it's Raman scattering, given that inelastic Raman scattering is typically weak (of course the amplitudes could be strong in the specific material) – Wikipedia says: "Rayleigh scattering usually has an intensity in the range 0.1% to 0.01% relative to that of a radiation source. An even smaller fraction of the scattered photons (approximately 1 in 1 million) can be scattered inelastically, with the scattered photons having an energy different (usually lower) from those of the incident photons—these are Raman scattered photons." https://en.wikipedia.org/wiki/Raman_scattering – Sebastian Riese Dec 08 '22 at 21:24