Why does light slow down when entering a prism but speed up when it exits? Where does it get the energy to increase it's velocity?

Asked by: Vladimir Berkov

Answer

The light waves that go through the glass don't actually slow down. The effect is only apparent and applies to the speed of light 'in the material' as opposed to the speed of light 'in vacuum' where light ALWAYS travels at the speed of light c.

The reason you treat the light as if it did slow down is an effect of the wave nature of light. We treat light as a wave, with a wavefront propagating with velocity c in vacuum. The wavefront represents a plane in which all the light waves are in the same phase. Now, when this wavefront hits a material, some of the wavelets will hit atoms and excite electrons to a higher energy state. Effectively, the electrons are 'swallowing' the light photon. Every material does this in a different way. The excited electron soon after releases the stored energy in the form of another photon. The key idea here is what happens to the phase of the wave as it gets absorbed and re-emitted. Depending on the resonant or natural frequency of the atom and the frequency of the incoming wave, the emitted photon will have changed phase when compared to it's unaffected brethren. It falls either back of forward a bit. The wavelet will do this every time it hits an atom, and there are quite a bit of atoms in even a small piece of material. This has the effect of retarding (or advancing) the wavefront as the wavelets go through the substance. The effect is most pronounced when the incoming waves are near, but not at, the resonant frequency of the material. At these frequencies, the change in phase lag (or change in effective wave speed depending on how you look at it) is great for a given change in wavelength. Most materials will have the effect of slowing the speed of the wavefront, but plasmas will actually speed it up. Notice the light wave is still only propagating at c, the phase velocity of the wave, however, may travel less, or even greater than at the speed of light.
Answered by: Jesse Clair, Physics Undergrad, Mount Allison U, Sackville, NB