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 black body) 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

 or light quantum

Minute 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.

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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.