Cerenkov radiation, also known as the Cerenkov effect, is continuous electromagnetic radiation emitted by particles travelling faster than the speed of light in a material. It was first described by Russian physicist Pavel Cerenkov, who earned the 1958 Nobel Prize for his contribution. The phenomenon is often observed as a bluish glow in some types of nuclear reactors, due to high-energy electrons travelling through water.
The Cerenkov effect is not a violation of special relativity, which states that no particle can travel faster than the speed of light in vacuum. The speed of light in materials is less than the speed of light in vacuum, and particles may travel at velocities greater than the speed of light in a particular medium but slower than the speed of light in vacuum.