Cherenkov radiation or Cerenkov radiation [for P. A. Cherenkov], light emitted by a transparent medium when charged particles pass through it at a speed greater than the speed of light in the medium. The effect, discovered by Cherenkov in 1934 while he was studying the effects of gamma rays on liquids and explained in 1937 by I. E. Tamm and I. M. Frank, is analogous to the creation of a sonic boom when an object exceeds the speed of sound in a medium. The light is emitted only in directions inclined at a certain angle to the direction of the particles' motion dependent upon the particles' momentum. Thus, by simply measuring the angle between the radiation and the path of the particles, the particles' speed may be determined. The effect is used in the Cherenkov counter, a device for detecting fast particles and determining their speeds or distinguishing between particles of different speeds.