Cherenkov Radiation

Cherenkov radiation is the blue glow that is often seen radiating off of nuclear reactors as shown above. It occurs when a charged particle (in the form of beta radiation) is emitted from a nuclear reactor travelling faster than the speed of light in the medium surrounding it. In the case of a nuclear reactors this medium is generally water.

In water the speed of light is lower than the speed of light in a vacuum - it is 2.25x10^8m/s compared to 3x10^8. This means that if a particle is emitted from the nuclear reactor it is moving faster than the speed of light in water. As these particles generally travel very fast and are polar they are not refracted instantly, and this means that you can end up with particles of beta radiation (which are electrons) travelling faster than the speed of light in the water.  This means that as the electron travels it partially polarises the water and causes a disruption in the electromagnetic field. At lower velocities the field can respond elastically and not much happens, but at such high velocities the disturbance cannot respond quickly, and a shockwave of light builds up.

Cherenkov Radiation is blue because the the majority of the light given out is high energy. This means that it has high frequencies (E=hf) and to our eyes the blue cones are much more receptive to blue light than violet light. This means that the light appears blue to us, and not violet.

Sources;

http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/cherenkov.html

http://math.ucr.edu/home/baez/physics/General/BlueSky/blue_sky.html