What is Cerenkov Radiation?
Like sonic boom in air and bow wave in water, shock wave also occurs in light.
We all know that the speed of light in vacuum is the speed limit. However, light propagates in a lower speed in media such as water. Here the speed of light is reduced to 75% of its speed in vacuum. Hence, it is possible that matters such as high-energy charged particles move faster than light in the medium of water. When this happens, electrons of the medium atoms may be excited and instantaneously emit weak electromagnetic wave (or photons, which is simply light). The electromagnetic wave thus generated will catch up the ones emitted earlier and become amplified. This forms an electromagnetic shock wave known as the Cerenkov Radiation.
The blue glow coming out from the tank of water in a nuclear reactor shown in Figure 1 is not fluorescent light, but is Cerenkov Radiation. Since it is simply visible light, it is not radioactive.
Figure 1 (Image source: Wikimedia Commons )