Why Electromagnetic waves travels at same speed in vacuum, but not in glass?

Question

Why Electromagnetic waves travels at same speed in vacuum, but not in glass?

Answer 1

Electromagnetic waves travel al the same speed in vacuum because photons have zero rest mass, but why that is so, I don't think is known. In glass they travel at different speeds, depending on the wavelength. The reason is that as light propagates it polarizes the atoms of the glass, and because of the inertia of the electrons, the polarization wave lags a little behind the light wave.  You have to think of the two forming some kind of collective wave, or disturbance.  Between the atoms the light propagates with the speed c, its vacuum speed, but each time it hits an atom it slows down a bit.  So the net effect is that the disturbance as a whole goes a little  slower.  Now, the reason why in glass light of shorter wavelength (higher frequency) travels slower than that of longer wavelength is because the higher frequencies are closer to the natural vibration frequencies of the electrons, so the interaction is stronger.

Answer 2

A simple partial answer is this: As the light strikes the molecules in the glass, the photons are absorbed, re-emitted, re-absorbed, and so on. This takes a small but finite time. And that time depends on the photon energy and therefore the wavelength.