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(fō`tŏn), the particle composing light and other forms of electromagnetic radiationelectromagnetic radiation,
energy radiated in the form of a wave as a result of the motion of electric charges. A moving charge gives rise to a magnetic field, and if the motion is changing (accelerated), then the magnetic field varies and in turn produces an electric field.
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, 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 blackbodyblackbody,
in physics, an ideal black substance that absorbs all and reflects none of the radiant energy falling on it. Lampblack, or powdered carbon, which reflects less than 2% of the radiation falling on it, crudely approximates an ideal blackbody; a material consisting of a
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) and the photoelectric effectphotoelectric effect,
emission of electrons by substances, especially metals, when light falls on their surfaces. The effect was discovered by H. R. Hertz in 1887. The failure of the classical theory of electromagnetic radiation to explain it helped lead to the development of
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. 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 effectCompton effect
[for A. H. Compton], increase in the wavelengths of X rays and gamma rays when they collide with and are scattered from loosely bound electrons in matter.
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. See quantum theoryquantum theory,
modern physical theory concerned with the emission and absorption of energy by matter and with the motion of material particles; the quantum theory and the theory of relativity together form the theoretical basis of modern physics.
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An entity that can be loosely described as a quantum of energy of electromagnetic radiation. According to classical electromagnetic theory, an electromagnetic wave can transfer arbitrarily small amounts of energy to matter. According to the quantum theory of radiation, however, the energy is transferred in discrete amounts. The energy of a photon is the product of Planck's constant and the frequency of the electromagnetic field. In addition to energy, the photon possesses momentum and also possesses angular momentum corresponding to a spin of unity. The interaction of radiation with matter involves the absorption, scattering, and emission of photons. Consequently, the energy interchange is inherently quantized. See Angular momentum, Energy, Momentum, Spin (quantum mechanics)

For many purposes, the photon behaves like a particle of zero rest mass moving at the speed of light. The particlelike nature of the photon is vividly exhibited by the photoelectric effect, predicted by A. Einstein, in which light is absorbed in a metal, causing electrons to be ejected. An electron absorbs a photon, gaining its energy. In leaving the metal, it loses energy because of interactions with the surface; the energy loss equals the product of the so-called work function of the surface and the charge of the electron. The final kinetic energy of the electron therefore equals the energy of the incident photon minus this energy loss. See Photoemission

A second demonstration of the particlelike behavior of photons is provided by the scattering of an x-ray photon from an electron bound in an atom. The electron recoils because of the momentum of the photon, thereby gaining energy. As a result, the frequency, and hence the wavelength of the scattered x-ray, is altered. If the x-ray is scattered through a certain angle, the wavelength is shifted by an amount determined by this scattering angle and the mass of an electron, according to the laws of conservation of energy and momentum. See Compton effect

From a more fundamental view, the photon is the quantum of excitation of a single mode of a radiation field. The dynamical equations for the electric and magnetic energy in such a field are identical to those of a harmonic oscillator. According to quantum theory, the allowed energies of a harmonic oscillator are given by E = ( j + ½)hf, where h is Planck's constant, f is the frequency of the oscillator, and the quantum number j = 0, 1, 2, …, describes the state of excitation of the oscillator. This quantum relation was first postulated by M. Planck for the material oscillators in the walls of a thermal enclosure in order to obtain the correct form for the density of radiation in a thermal field, but it was quickly applied by Einstein to describe the state of the radiation field itself. In this picture, j describes the number of photons in the field. See Harmonic oscillator, Quantum electrodynamics, Quantum mechanics


(foh -ton) Symbol: γ. A quantum of electromagnetic radiation. The photon can be considered as an elementary particle with zero rest mass, charge, and spin, traveling at the speed of light. A beam of electromagnetic radiation can thus be thought of as a beam of photons, with the intensity of the radiation proportional to the number of photons present. The energy of the photons, and hence of the radiation, is equal to the product, h ν, of the Planck constant and the frequency of the radiation. Photons with sufficient energy can ionize atoms and disintegrate nuclei. Although photons have no rest mass they do have momentum and thus exert a pressure, usually referred to as radiation pressure.



an elementary particle, the quantum of electromagnetic radiation; in a narrow sense, a quantum of light.

The rest mass m0 of the photon is equal to zero (experimental data indicate that, in any event, m0 ≤ 4 × 10–21me, where me is the mass of the electron). Therefore, the velocity of the photon is equal to the speed of light c ≈ 3 × 1010 cm/sec. The spin, or intrinsic angular momentum, of the photon is equal to 1 in units of ℏ = h/2π, where h = 6.624 × 10–27 erg-sec is Planck’s constant. Therefore, the photon is a boson. A particle with spin J and a nonzero rest mass has 2J + 1 spin states that differ in the projection of the spin. Because the photon has m0 = 0, it can have only two spin states, + 1 and –1, with the projection of the spin onto the direction of the particle’s motion; in classical electrodynamics, the transverse nature of an electromagnetic wave corresponds to this property of the photon.

Since there is no frame of reference in which the photon is at rest, no specific intrinsic parity can be assigned to the photon. Depending on whether the system of charges, or the 2’-pole, that emitted a given photon is an electric multipole or a magnetic multipole (seeMULTIPOLE), a distinction is made between a photon of electric-multipole radiation and a photon of magnetic-multipole radiation. The parity of a photon of electric-multipole radiation is equal to (–1)l the parity of a photon of magnetic-multipole radiation is equal to (–1)l +1. The photon is a truly neutral particle and, therefore, has a certain charge parity, which is equal to –1 (seeCHARGE CONJUGATION). In addition to the electromagnetic interaction, the photon participates in the gravitational interaction.

The concept of the photon arose during the development of quantum theory and the theory of relativity. (The term “photon” appeared only in 1929.) In 1900, M. Planck obtained an equation for the thermal radiation spectrum of a blackbody on the assumption that electromagnetic waves are radiated in specific portions, called quanta, whose energy may take on only a discrete set of values that are multiples of an individual portion—the quantum hv, where v is the frequency of the electromagnetic wave. Developing Planck’s idea, A. Einstein introduced the hypothesis of light quanta, according to which the discreteness is due not to the mechanism of absorption and emission but to the fact that radiation consists of “indivisible quanta of energy that are absorbed or emitted only in their entirety” (Sobr. nauch. trudov, vol. 3, p. 93, Moscow, 1966). The hypothesis enabled Einstein to explain a number of regularities of the photoelectric effect, luminescence, and photochemical reactions. At the same time, the special theory of relativity, which was developed by Einstein in 1905, led to the rejection of the explanation of electromagnetic waves as vibrations of a special medium, called the ether, and thus established the premises for considering radiation as a form of matter and light quanta as real elementary particles. A. Compton’s X-ray scattering experiments showed that quanta of radiation obey the same kinematic laws as do particles of matter. In particular, a momentum hv/c must also be assigned to a quantum of radiation of frequency v (seeCOMPTON EFFECT).

By the mid-1930’s, the development of quantum mechanics made it clear that neither the existence of wave properties, as manifested in the wavelike nature of light, nor the ability to disappear and appear in absorption and emission events distinguishes photons from other elementary particles. It turned out that particles of matter, such as electrons, have wave properties (seeDE BROGUE WAVES and DIFFRACTION OF PARTICLES). Also, the possibility of the mutual transformation of electron-positron pairs and photons was established (seeANNIHILATION AND CREATION OF PARTICLE-ANTIPARTICLE PAIRS). For example, in the electrostatic field of an atomic nucleus, a photon with an energy higher than 1 million electron volts (eV) may be transformed into an electron and a positron in the process known as pair production; conversely, a collision of an electron and a positron results in the transformation of the particle-antiparticle pair into two or three gamma rays in the process called pair annihilation (photons with an energy higher than 100,000 eV are often referred to as gamma rays).

Quantum electrodynamics is a recent theory that consistently describes the interactions of photons, electrons, and positrons with allowance for possible mutual transformations (seeQUANTUM FIELD THEORY). In quantum electrodynamics, the electromagnetic interaction of charged particles is considered as the exchange of virtual photons (seeVIRTUAL PARTICLES). In addition, photons may interact with one another through the creation of virtual electron-positron pairs. However, the probability of such an interaction is very low. In the scattering of high-energy photons by hadrons and atomic nuclei, the possibility should be taken into account that a photon may undergo a virtual transformation into a set of hadrons that strongly interact with the target hadrons. A virtual photon that is created in, for example, the annihilation of a high-energy electron and high-energy positron may be transformed into real hadrons. (Such processes are observed in colliding electron-positron beams.) The interaction of real and virtual photons with hadrons is described by means of various theoretical models, such as vector-meson dominance and parton models.

A unified theory of the electromagnetic and weak interactions has been developed since the late 1960’s. The unified theory postulates that the photon acts together with three hypothetical carriers of the weak interaction, which are called vector bosons; the existence of two charged vector bosons, W+ and W, and one neutral vector boson, Z0, is assumed.

Light sources are well-known sources of photons. Radioactive isotopes and targets bombarded by accelerated electrons are sources of gamma rays.


Einstein, A. “O razvitii nashikh vzgliadov na sushchnost’ i strukturu izlucheniia.” Sobr. nauch. trudov, vol. 3. Moscow, 1966. Page 181.
Bohm, D. Kvantovaia teoriia, 2nd ed. Moscow, 1965. (Translated from English.)



(quantum mechanics)
A massless particle, the quantum of the electromagnetic field, carrying energy, momentum, and angular momentum. Also known as light quantum.


a quantum of electromagnetic radiation, regarded as a particle with zero rest mass and charge, unit spin, and energy equal to the product of the frequency of the radiation and the Planck constant


A quantum of electromagnetic energy. Like electrons, photons appear as both waves and particles at the same time. Quite often, a photon is said to be a "particle of light;" however, radio transmission, X-rays and gamma rays are also made up of particles. Although they may not always be called photons, they are the same phenomena at different frequencies.

The energy of an individual photon is proportional to its frequency, which is why a single photon of light has more energy than a photon in the radio spectrum below it. A single light photon can cause a neuron in your retina to fire or convert silver iodide to silver and iodine on photographic film. However, a single radio photon is nearly impossible to detect, and all by itself, is not doing anything that we want to measure. See photoelectric, photonic and wave-particle duality.

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