[Suicide Junkie]

Originally posted by Imperator Fyron:
No, light has constant velocity. Different wavelengths travel at marginally different speeds, but it is all constant. Light seeming to slow down in water and such is an optical illusion.

Same thing.
The initial wavefront moving thorugh the material goes right through at the speed of light.
The interference from the secondary EM waves generated by the atoms jostled by the main wave adds up to produce a new wave whos peaks and valleys change position slower than you'd expect.

Look at standing waves on a rope as a slowed down example. You start waving the rope (attached to a wall or a friend on the other side)
You see a succession of waves travel down the rope, hit the wall, and then reflect back, and start to interfere with the later waves you made.
When the two waves interfere in the rope, the total result is a wave that looks like it isn't moving at all.
The energy is still moving along at the normal speed, but the interference causes the illusion of slower (or faster (!)) velocities.
Given appropriate conditions, you can actually make the wave peaks appear to "travel" faster than C, but the far side of the apparatus will not light up until enough time has passed for a beam at speed C to travel the length of the apparatus.