When a charged particle travles faster than the speed of light in a (dielectric) medium (less than the speed of light in a vacuum, of course) it creates light, like a sonic boom in air. The particle polarizes the molecules it passes, and to realign themselves, the molecules emit light. When the particle is moving faster then light, constructive interference preserves the light - if not, destructive interference destroys it.

Causes radioactive things to glow blue underwater -Nifty