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photon

photon (fō´tŏn), the particle composing light and other forms of electromagnetic radiation, sometimes called light quantum. The photon has no charge and no mass. About the beginning of the 20th cent., the classical theory that light is emitted and absorbed by matter in a continuous stream came under criticism because it led to incorrect predictions about several effects, notably the radiation of light by incandescent bodies (see blackbody) and the photoelectric effect. These effects can be explained only by assuming that the energy is transferred in discrete packets, or photons, the energy of each photon being equal to the frequency of the light multiplied by Planck's constant, h. Because the value of Planck's constant is extremely small (6.62 × 10-27 erg sec.), the discrete nature of light energy is not evident in most optical phenomena. The light imparts energy and momentum to a charged particle when one of the photons collides with it, as is demonstrated by the Compton effect. See quantum theory.

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photon

photon Quantum of electromagnetic radiation, such as light; a ‘particle’ of light. The energy of a photon equals the frequency of the radiation multiplied by Planck's constant. Absorption of photons by atoms and molecules can cause excitation or ionization. A photon may be classified as a stable elementary particle of zero rest mass, zero charge, spin 1, and travelling at the velocity of light. It is its own antiparticle. Virtual photons are thought to be continuously exchanged between charged particles and thus to be the carriers of electromagnetic force (potential difference between terminals in a source of electric current). See also quantum theory

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photon

pho·ton / ˈfōtän/ • n. Physics a particle representing a quantum of light or other electromagnetic radiation. A photon carries energy proportional to the radiation frequency but has zero rest mass.

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photon

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