# Does a photon's energy change as it refracts?

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• maxelcat
maxelcat
TL;DR Summary
Does the energy of a photon change as its velocity changes due to refraction?
I believe that when a photon is refracted it slows. Why is it that the wavelength decreases but that its frequency stays constant?

Does this imply that the photon has not lost any energy in the process of slowing down given that E=hf?
Thanks

maxelcat said:
Does this imply that the photon has not lost any energy in the process of slowing down given that E=hf?
Yes, this is the implication. If a photon transfers energy somewhere, then either the photon vanishes completely, or its frequency goes down.

But I answered, because I thought about the same question in the context of electrons in a crystal recently, and the eigenvalue problem one solves to get the Bloch waves. I wondered whether the eigenvalues are related to the energy, or to something else. Initially I was convinced that it must be related to energy, but in the last few days I changed my mind, because the frequency cannot really change, without an interaction and energy transfer.

maxelcat said:
TL;DR Summary: Does the energy of a photon change as its velocity changes due to refraction?

I believe that when a photon is refracted it slows. Why is it that the wavelength decreases but that its frequency stays constant?

Does this imply that the photon has not lost any energy in the process of slowing down given that E=hf?
Thanks
If the material is transparent and finite, you can say that the photons entering the material and leaving the material have the same energy. However the photons inside the material are no longer photons but quasi-particles of the light-matter system (you can be sure of that because the wave vector ##\mathbf k## is no longer the same, so your energy dispersion relation is not that of a free photon).

Part of the incident energy is reflected when a wave strikes a transparent medium, so I presume that in quantum terms an occasional photon is reflected at the surface?

tech99 said:
Part of the incident energy is reflected when a wave strikes a transparent medium, so I presume that in quantum terms an occasional photon is reflected at the surface?
Yes. If you look at the whole energy balance, you will lose some energy of the light beam in reflection, diffusive scattering and heat, but if you look at the individual photons that exit the material, these photons have usually the same frequency (same for those that are specular reflected).

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