It is well known
that the speed of light represents the upper limit of velocity for all
energy and matter in the universe. However, the speed at which light
travels changes between mediums, for a vacuum it
measures c, or
roughly 3x10^8 m/s. In another medium, such as water, the speed of
light can be significantly slower.
Now, it is possible to accelerate a
particle through a medium faster than the speed of light in that
medium. When this is done the molecules in that medium are
when returning to their ground state they emit radiation.
The blue glow that is
attributed to Cherenkov Radiation