Cherenkov

Cherenkov radiation is the electromagnetic radiation emitted when a charged particle travels through a dielectric medium at a speed exceeding the phase velocity of light in that medium. It was observed by Pavel A. Cherenkov in 1934, and its theoretical interpretation was completed by Ilya Frank and Igor Mueller; the trio received the Nobel Prize in Physics in 1958 for the discovery and its explanation.

The emission arises from the polarization of molecules in the medium as the fast-moving charge disturbs the local electromagnetic field. The radiation forms a cone with opening angle θ determined by the relation cos θ = 1/(β n(λ)), where β = v/c and n(λ) is the medium’s refractive index for light of wavelength λ. Cherenkov light is produced only if β n(λ) > 1, and its spectrum is broad, typically more intense at shorter wavelengths, giving a characteristic blue glow in water or other transparent media.

Detection and applications rely on the directional light produced by Cherenkov radiation. Photomultiplier tubes or other light sensors surrounding the medium capture the light to identify and study high-energy charged particles. Large-scale detectors using water or ice, such as Super-Kamiokande and IceCube, are used in neutrino astronomy and particle physics. The phenomenon also informs reactor monitoring, radiation dosimetry, and certain medical imaging techniques.