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I have been taught in my school (and read on many internet sites too) that frequency of light doesn't change by changing the medium.

Also I read that the colour we see depends on the energy of the photons (which depends on the frequency of the photon) reaching our eye in this answer. So we can conclude that when medium is changed colour of light doesn't change.

Now see this :

enter image description here

The heart shaped structure is actually a paperweight of glass. When I passed the green laser from it the colour of the light beam changed to red which means that the frequency changed (more specifically : the frequency decreased) on changing the medium.

Now this is what I want to know. How did the medium changed the colour of the light ?

Why is the picture in contradiction with the theory?

Ankit
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    I would like to add that this is not happening at a particular angle. It is generally true for all angles of incidence – Ankit Jun 06 '21 at 15:31
  • The red light is not a reflection of any other light .. I can even add pictures in complete darkness if needed. – Ankit Jun 06 '21 at 16:03
  • Do you have any information what kind of glas that is? – Charles Tucker 3 Jun 06 '21 at 16:12
  • @Charles Tucker 3 sorry I bought it from my friend.. – Ankit Jun 06 '21 at 16:13
  • Green glass is generally coloured using iron (II) oxide, plus some chromium compound (probably oxide) See https://en.wikipedia.org/wiki/Glass_coloring_and_color_marking Unfortunately, that article doesn't mention the oxidation state of the chromium, which has quite a few. FWIW, plain clear glass usually has a slight greenish tinge, due to traces of iron. – PM 2Ring Jun 06 '21 at 17:18
  • @PM 2Ring that is not a green glass I guess... – Ankit Jun 06 '21 at 17:28
  • Oh. What colour does it have in sunlight? – PM 2Ring Jun 06 '21 at 17:29
  • @PM 2Ring its perfectly transparent.. so I guess it's white or somewhat silvery .(in sense of transparency) – Ankit Jun 06 '21 at 18:08
  • Ok. If it's perfectly clear (even when you look through the side), there's a possibility that it was decolourised using cerium oxide. See https://en.wikipedia.org/wiki/Cerium(IV)_oxide#Optics – PM 2Ring Jun 06 '21 at 18:28
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    Just a comment to give a simple intuitive feeling of the phenomenon. If you ever played with UV lights and fluorescent paints, or even just a plain old white t-shirt, you see that the UV light makes these materials glow. You are observing a very similar effect with the laser and the glass. The green light excites the material and it then radiates at higher wavelength. – José Andrade Jun 08 '21 at 11:59
  • Can you see this redlight with your eye? Many green lasers emit green light and infrared light which often have very different absorption spectrum in glass – shai horowitz Jul 11 '21 at 12:54
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    @shai horowitz trust me I can see them with my eyes.. but I don't know what can I do so that you guys can believe.. – Ankit Jul 11 '21 at 13:26
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    We believe you. Were just pointing out hypothesis that can be easily shot down to narrow the playing field – shai horowitz Jul 11 '21 at 14:25

1 Answers1

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Assuming everything is correct, and this red light is not a reflection of some other light source or something: one possible way of achieving this is that the medium is absorbing green light and emitting the red light through photoluminescence. So in this case it's technically not light changing frequency in the medium; it's "new" light produced by the medium, and it is characteristic for the particular medium. In short, if the medium absorbs green light and emits red in this process, it means that there is an allowed optical transition with energy corresponding to the green light, and then this excited state is relaxed to a lower-energy state. The latter can then relax to the ground state emitting a red photon. So technically, the spectrum of the photoluminescence contains information about the energy levels in the medium.

Note that in linear optics (single photon processes only) it can only go one-way (green to red in your example) because the energy of the green photon is higher. So the conservation of energy is not violated, and some energy is lost in the medium (e.g. on heating). This means that if you get a source of red light and shine its light on the same glass, it won't produce green light. In nonlinear optics, this is not true; for example, two photons with energies half of that of the green photon can be absorbed, and one red photon can be emitted, bit for that you need high intensity (laser light could work actually).

Of course, when I write "red" or "green" photon, I mean a photon with the energy corresponding to the wavelength of green or red light, respectively.

sleepy
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  • color perception could play a role too, http://hyperphysics.phy-astr.gsu.edu/hbase/vision/colper.html . i.e. the green giving rise to a cascade of new photons that add up to a perception of red. – anna v Jun 06 '21 at 17:30
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    @annav not in this case. The picture clearly shows fluorescence. – José Andrade Jun 08 '21 at 11:56