Insights Do Photons Move Slower in a Solid Medium? - Comments

  • Thread starter ZapperZ
  • Start date
Some questions come to my mind.
1. Can you try to explain how a mirror actually works?
2. Can you expand a bit on translucent materials? Why do some photons get absorbed and others don't? Is the absorption probabilistic, or can it be understood as a cloud of opaque particles in transparent media?


Science Advisor
I don't think that phonons are that important to explain dispersion of light in the visible region as addressed in this article. Much more important are electronic excitations of the atoms and molecules making up the substance. If these excitations are coupled, one speaks of excitons in analogy to photons. In deed the electrons have a much larger oscillator strength as compared with the phonons.
Although in transparent media like glass, the eigenfrequencies -or rather broad absorption bands - of these modes lie in the ultraviolet region of the spectrum, they can influence the propagation of light in the visible region. The point is that, though nonresonant, the electric field of the light wave can drive forced polarization of the electron clouds. Now ,- well below the resonance frequency - while the phase of the polarisation will be in phase with the driving field, the electric field radiated by the polarisation will lag behind by 90 degrees. This increasing phase lag is nothing else but a reduction of phase velocity.


Gold Member
Correct me if I’m wrong but I think that the phonon polariton model described in this article is dominant only at very far infrared frequencies. I believe the dipole interaction with bound electrons becomes dominant at optical frequencies. (I thought a dressed photon state was called a polariton, not an exciton, by the way.)

It might be worth mentioning therefore that this article describes one example of the various ways photons interact with collective excitations?


Science Advisor
Daz, you are obviously right, I wanted to say "in analogy to phonons" not photons. One can speak of exciton-polaritons and phonon-polaritons.

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