I'm learning optics and have been told that when light enters a medium (e.g. glass) and slows down the frequency of the light stays constant while it is the wavelength which is reduced. The explanation for this is that there is no energy loss from the beam of light passing through the glass and since $E=h\nu$ we have the same frequency and that we can easily verify this by looking at a sigle frequency light source through glass and noting the colour doesn't change.

I've also been told that the reason that light slows down in glass is because the Electromagnetic field of a beam of light interacts with and accelerates the charged electrons in the glass molecules which then generate their own photons and the combined wave resulting from the beam and these photons have a slower group velocity, hence the light travels slower.

Both of these explanations make sense on their own, however since the second explanation requires the wave to accelerate electrons that means the wave must give up some of its own energy to these electrons. That would then imply the loss of energy in the wave itself and since $E=h\nu$ we'd see the frequency of the wave shift and thus the colour of the wave would look different when we observed it. However that's very much not the case empirically so where is my intuition going wrong?

  • $\begingroup$ What about the light being absorbed by an atom and then re-radiated at the same frequency? $\endgroup$
    – Farcher
    Commented Apr 18 at 15:40

1 Answer 1


Your explanation of refraction is quite correct. The electric field of the light interacts with the electrons in the medium and we get an entangled system of light + electrons. This means the energy of the light is shared with the electrons.

However when the light leaves the medium again the electrons at the edge of the medium reradiate the energy as light. In effect the light loses some energy to the electrons when it enters the medium then gets the energy back again when it leaves.

  • $\begingroup$ In that case why doesn't the colour of the light/frequency change while it's in the dense medium compared to outside the medium? $\endgroup$
    – Hadi Khan
    Commented Apr 18 at 16:22
  • $\begingroup$ Because you see the colour once the light has left the glass and got all its energy back. $\endgroup$ Commented Apr 18 at 16:23
  • $\begingroup$ I think the measured frequency of the light is still the same even in cases where we have a detector which is encased within the glass completely. Plus we can consider the case where instead of glass I open my eyes underwater while swimming with no goggles. There's no phase transition in that case at all but the colours I see still aren't shifted compared to if the pool was empty. $\endgroup$
    – Hadi Khan
    Commented Apr 18 at 16:39
  • 2
    $\begingroup$ @HadiKhan there are transitions though. The light leaves the water, enters the lens of the eye, then the vitreous humor, then hits the light sensing cells on your retina....by that time it's definitely left each medium it was in previously. $\endgroup$
    – Triatticus
    Commented Apr 18 at 16:57
  • $\begingroup$ To make things simpler I think we can focus on a piece of plastic coated with a fluorescent paint that absorbs light at 300nm and emits at 500nm (say). If I have this and I shine a 300nm source on it in the air it will flouresce. If I then were to move this piece of plastic underwater and shine the UV light on it then I'd still expect it to fluoresce, even though if the frequency of the UV light has now changed the paint shouldn't be able to absorb the light any more. $\endgroup$
    – Hadi Khan
    Commented Apr 18 at 17:02

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