As I see it, which is probably wrong, the speed of light changes in different media because of absorption and re-emission of the photons. This sequence of events takes longer than light just passing straight through a vacuo, which means that the average velocity will be lower. I have a feeling, though, that this is wrong. If absorption and re-emission is what happened even in transparent substances, you would see a very fuzzy picture through them because re-emission has no directional preference - the image will be completely dispersed (which it isn't). So why do different media slow light down?