The Cerenkov effect is the emission of light from a transparent substance like water or glass when a charged particle, such as an electron, travels through the material with a speed faster than the speed of light in that material.
The Cerenkov effect was discovered by Russian experimentalist P. A. Cerenkov in 1934 and explained by Russian theorists I. Y. Tamm and I. M. Frank. All three scientists received the Nobel prize in physics in 1958.
The electric field of a fast-moving charged particle shifts the electrons of the atoms of a nonconducting material as the particle passes through. When the particle travels at speeds faster than the speed of light in the material, the atoms respond by emitting light in a cone at an angle determined by the index of refraction of the material. The process can be compared to that of a shock wave of sound generated when an airplane exceeds the speed of sound in air.
The index of refraction is computed by dividing the speed of light in a vacuum (3x108 m/sec or 186,000 mi/sec) by the speed of light in the medium through which the particle passes. The speed of light in lucite or heavy lead glass, for example, is 2x108 m/sec; therefore the index of refraction for those substances is 1.5.
Cerenkov detectors use the properties of Cerenkov radiation in high energy physics and cosmic ray physics experiments. Since the radiation is only emitted when the velocity of the particle is above a predetermined speed, a "threshold" value for the particle velocity can be set on the detector to discriminate against slow particles. For a particle velocity above the threshold value, an angular measurement of the Cerenkov light relative to the particle direction determines the velocity of the particle.