[neutralino]

Originally Posted by NotStein

But in glass it does, otherwise a prism wouldn't work; water, too, otherwise spearing fish would be much easier. Different configurations and densities of matter (mass, and therefore energy, too) affect the speed of photons. Wouldn't then the configuration/density of the fabric of space be responsible for fixing the universal speed of photons (C)? If not, what would (something does). Or are you saying that all of this is simply due to curvature, and velocity remains constant?

Jeff

Note my careful use of words in the above post. I said that the speed of a photon does not change, whereas the speed of the light through the medium appears to change.

To explain this phenomenon, we need to go back to basics. We imagine a solid medium as being made up of ions and electrons in a lattice structure. We know that these ions can vibrate (since this corresponds to the increase in kinetic energy of the solid). Now, the vibrational modes of the lattice are commonly called phonons: these phonons are the quanta of the vibrational modes. When a photon meets a solid, there are two things that can happen. Either it has an energy the same as one phonon mode; in which case it will interact with a phonon and then be absorbed and turned into heat. Or, the photon has an energy beyond the phonon spectrum. In this case the photon can disturb the lattice, but it cannot keep the vibrations up, since there is no available phonon mode. This photon is not absorbed and is emitted, but with some delay. As the photon continues to move down the medium, the same thing happens many times. It is this that gives us the apparent slowing of the light speed in the material.