Cherenkov Radiation in the Reed Research Reactor (United States Nuclear Regulatory Commission)

Light travels at a constant speed, roughly 3.0×108 m/s. But when light travels through a material such as glass or plastic, it has a lower effective speed because the photon interacts with the electric fields of atoms an molecules along the way through a number of quantum mechanical effects. When a high energy charged particle such as an electron travels through a dielectric material (a material that contains molecules that can be polarized) at a velocity higher than that of the effective speed of light, it creates Cherenkov Radiation, resulting in a bluish glow of light. This happens because the electromagnetic field of the electron causes a slight and temporary polarization of the molecules in the surrounding medium. This distorts the electric fields of the molecules, which return to their original shape after the electron passes. The rebound in the molecule creates a new electromagnetic wave (light). Nuclear reactors typically emit such high energy electrons, so it is possible to see in the image of a nuclear reactor here.

Wikipedia: Cherenkov Radiation