[Fyron]

Light seems to slow down because the different wavelengths travel at slightly different speeds. When a beam white light (which consists of all colors/wavelengths of visible light) goes from a thin medium to a thick medium (such as from air to water), the faster wavelengths of light seem to have a longer distance to travel, as they are on the outside of the angle of refraction. The slower wavelenghts are on the inside of the turn, and so seem to travel a shorter distance. This gives the appearance of the light slowing down, when in actuality, it does not. I forget the exact mathematical proof that shows that the light did not slow down any.