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A photon is the quantum of energy in the form of electromagnetic radiation, emitted or absorbed by matter.

A photon is an elementary particle, the quantum of all forms of electromagnetic radiation, including light. It is the particle mediating the electromagnetic force, even when it is static through virtual photons.

The photon has zero mass at rest and consequently the interactions of this fundamental force are observable at both microscopic and macroscopic scale.

Like all elementary particles, photons are explained by quantum mechanics but exhibit wave-particle duality, simultaneously exhibiting properties of waves and particles. For example, a lens may refract a single photon and in the process interfere with itself as if it were a wave, or it may act as a particle having a definite position and a measurable amount of motion.

The wavelength and quantum properties of the photon are two observable aspects of the same phenomenon and its nature can not be described in terms of any mechanical model, so the representation of this dual property of light, which assumes that energy Is concentrated in certain points of the wave front, is also impossible. Quanta in a wave of light can not be located in space; Some definite physical parameters of the photon are noted.

The photon has spin equal to 1, and, therefore, it is a bosón; As its resting mass is zero, the helicity of the photon can only be 1 or -1, but not 0.

The photon is represented by the symbol γ.

History of the photon

The modern concept of the photon was developed gradually for Albert Einstein in the early twentieth century to explain experimental observations that did not agree with the classical model of light as an electromagnetic wave.

The photon model matched the fact that the energy of light depended on its frequency and explained the ability of matter and electromagnetic radiation to be in thermal equilibrium. In addition, the photon model also explained certain anomalous observations such as the black body radiation that other physicists, notably Max Planck, had attempted to explain using semi-classic models.

In the Planck model, light was described by Maxwell's equations, but material objects that emitted and absorbed light did so in discrete packets of energy. Although these semiclassic models contributed to the development of quantum mechanics, several subsequent experiments starting with the Compton effect validated the Einstein hypothesis that light itself is quantified.

In 1926 optical physicist Frithiof Wolfers and chemist Gilbert N. Lewis coined the term "photon" for these particles. After Arthur H. Compton won the Nobel Prize in 1927 for his scattering studies, most scientists accepted that quanta of light have an independent existence and accepted the photon name by these quanta.

The photon in particle physics

In the standard model of particle physics, photons and other elementary particles are described as a necessary consequence of the fact that the laws of physics have a certain symmetry in space-time. The intrinsic properties of the particles, such as electric charge, mass and spin are determined by the properties of this gauge symmetry.

The concept of photon has led to transcendent advances in theoretical and experimental physics, for example lasers, the Bose-Einstein condensate, quantum field theory, and the probabilistic interpretation of quantum mechanics. It has been applied in photochemistry, in high resolution microscopy and in the measurement of molecular distances. Recently, photons have been studied as an element of quantum computers and for their applications in optical imaging and optical communication such as quantum cryptography.

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Last review: March 13, 2017