Cherenkov Radiation
Linking supersonic booms to glowing nuclear facilities.
Similarly to the “boom” heard as an aircraft moves at supersonic speeds through air, Cherenkov radiation occurs due to charged paricles moving at, not supersonic, but superluminal (faster than the speed of light) speeds through a dielectric (poor electrical insulator, can be polarised electrically) medium.
Physics
Firstly, we note that the universal speed limit set by light is strictly true of light moving through a vacuum, however, within a medium it is entirely possible for matter to travel faster than light. As the charged particle moves through the dielectric medium, it excites the electrons in the nearby particles of that medium. When those electrons of nearby medium particles return to their ground state, they emit a photon wavefront in all directions. If the particle is moving through the medium faster than the speed of light in that medium, then these wavefronts will concentrate into a conical shape. Many particles doing this at any given time will give the appearance of a “glow” from their source within the medium. This is analagous to the sonic boom produced by something moving through air at supersonic speeds, as shown in Figure 1.
Example
The most ubiquitous example of Cherenkov radiation is that as a result of operational nuclear reactors housed within an open pool of liquid water. In such cases, high velocity electrons are released as products of nuclear fission which can move faster than the speed of light in water (0.75*c). Escaping the confides of the reactor, these electrons interact with the electrons of the water molecules to produce optical light (bluish in colour) - refer to Figure 2.
