Ok, I'm not absolutely sure about this. I don't have enough in depth knowledge of QM (quantum mechanics) to be sure, but here are my thoughts on the matter.
Whoops! There, I got myself so tangled up trying to think this through that I had to back up and start over.

I guess I could always say, because, but that isn't a very satisfying answer.

Originally Posted By: Wikipedia
At the microscale, an electromagnetic wave's phase speed is slowed in a material because the electric field creates a disturbance in the charges of each atom (primarily the electrons) proportional to the permittivity of the medium. The charges will, in general, oscillate slightly out of phase with respect to the driving electric field. The charges thus radiate their own electromagnetic wave that is at the same frequency but with a phase delay. The macroscopic sum of all such contributions in the material is a wave with the same frequency but shorter wavelength than the original, leading to a slowing of the wave's phase speed. Most of the radiation from oscillating material charges will modify the incoming wave, changing its velocity. However, some net energy will be radiated in other directions (see scattering).


So I copied this quote out of the Wikipedia article on Refractive index. What this says is a bit different from the idea that the atoms actually absorb and re-emit the photon. This seems to be saying that it absorbs some of the energy from the photon and re-emits it with a slight delay, so that the probability wave of the photon is pulled back slightly. That is a bit different from our usual idea of what happens when an atom absorbs a photon and the outer electron jumps up to a higher energy level. For a better understanding you might want to read the whole article.

Bill Gill


C is not the speed of light in a vacuum.
C is the universal speed limit.