..... Click the link for more information. , 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 black body black body, 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, approximates an ideal black body.
..... Click the link for more information. ) and the photoelectric effect photoelectric effect, emission of electrons by substances, especially metals, when light falls on their surfaces. The effect was discovered by H. R. Hertz in 1887.
..... Click the link for more information. . 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 Compton 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.
..... Click the link for more information. . See quantum theory quantum 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.
..... Click the link for more information. .
photon
or light quantumMinute energy packet of electromagnetic radiation. In 1900 Max Planck found that heat radiation is emitted and absorbed in distinct units, which he called quanta. In 1905 Albert Einstein explained the photoelectric effect, proposing the existence of discrete energy packets in light. The term photon came into use for these packets in 1926. The energies of photons range from high-energy gamma rays and X rays to low-energy infrared and radio waves, though all travel at the same speed, the speed of light. Photons have no electric charge or rest mass and are the carriers of the electromagnetic field.
photon
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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photon
photon [′fō‚tän]
Photon
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
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