Text: A charged particle emits Cerenkov radiation (light) in a cone around its direction of travel when it travels through any medium faster than the speed of light through that medium. (Cerenkov - is the name of the scientist who first recognized the nature of this effect and its possible use for distinguishing particle types.) Although the speed of light in a vacuum is the fastest speed that any particle or light can have, in a medium of any type light travels more slowly because of its interactions with the electric fields of the atoms in the medium and so it is possible for a high energy particle to be faster than light in some material . The blue light in the pools of water you may have seen in pictures of nuclear power plants is Cerenkov radiation from particles produced in the reactor.

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