= Cherenkov radiation
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= Cerenkov radiation
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{synonym}
= Vavilov-Cherenkov radiation
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<Cherenkov radiation> is <electromagnetic radiation> from a <charged particle> exceeding the medium's <phase velocity>: $c/n(\omega)<v<c$. Here $n(\omega)$ is the positive <refractive index> at <angular frequency> $\omega$. The threshold uses <phase velocity>, not <group velocity>.<Passage of Particles Through Matter by Donald E. Groom and Spencer Klein>[§34.7.1, equation (34.41), Fig. 34.26: phase-speed threshold]{ref}
The beauty of <Cherenkov radiation> is that uniform motion can generate a sharply directed <electromagnetic wave>: the medium's response along successive portions of the track adds with matching <wave phase> on a cone. The moving <electric charge> drives the medium's <electric polarization>; the resulting radiation depends on the collective <refractive index>. This differs from <luminescence>, where excitation of material is followed by <light> emission. Both mechanisms can contribute to the same irradiated sample. A visible blue glow alone therefore identifies neither <Cherenkov radiation> nor a particular radioactive source.
The <Cherenkov cone angle> turns a measured direction into a constraint on particle <speed> when the <refractive index> is known. The <Frank-Tamm formula> instead predicts the number of emitted <photons> in a specified <wavelength> interval. These are complementary observables: the <finite-band Cherenkov yield model> below shows why both depend on <optical dispersion> and on which <wavelengths> are collected. This makes <Cherenkov radiation> useful for <particle detectors>, with <photon> production, transport and detection treated separately.
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