In modern physics the photon is the elementary particle responsible for electromagnetic phenomena. It is the carrier of electromagnetic radiation of all wavelengths, including gamma rays, X-rays, ultraviolet light, visible light, infrared light, microwaves, and radio waves. The photon differs from many other elementary particles, such as the electron and the quark, in that it has zero rest mass;[3] therefore, it travels (in vacuum) at thespeed of light, c. Like all quanta, the photon has both wave and particle properties (“wave–particle duality”). As a wave, a single photon is distributed over space and shows wave-like phenomena, such as refraction by a lens and destructive interference when reflected waves cancel each other out; however, as a particle, it can only interact with matter by transferring the amount of energy
where h is Planck's constant, c is the speed of light, and λ is its wavelength. This is different from a classical wave, which may gain or lose arbitrary amounts of energy. For visible light the energy carried by a single photon would be around a tiny 
joules; this energy is just sufficient to excite a single molecule in a photoreceptor cell of an eye,[citation needed] thus contributing to vision.[4]
Apart from energy a photon also carries momentum and has a polarization. It follows the laws of quantum mechanics, which means that often these properties do not have a well-defined value for a given photon. Rather, they are defined as a probability to measure a certain polarization, position, or momentum. For example, although a photon can excite a single molecule, it is often impossible to predict beforehand whichmolecule will be excited.
The above description of a photon as a carrier of electromagnetic radiation is commonly used by physicists. However, in theoretical physics, a photon can be considered as a mediator for any type of electromagnetic interactions, including magnetic fields and electrostatic repulsion between like charges.
The modern concept of the photon was developed gradually (1905–17) by Albert Einstein[5][6][7][8] to explain experimental observations that did not fit the classical wave model of light. In particular, the photon model accounted for the frequency dependence of light's energy, and explained the ability of matter and radiation to be in thermal equilibrium. Other physicists sought to explain these anomalous observations by semiclassical models, in which light is still described by Maxwell's equations, but the material objects that emit and absorb light are quantized. Although these semiclassical models contributed to the development of quantum mechanics, further experiments proved Einstein's hypothesis that light itself is quantized; the quanta of light are photons.
The photon concept has led to momentous advances in experimental and theoretical physics, such as lasers,Bose–Einstein condensation, quantum field theory, and the probabilistic interpretation of quantum mechanics. According to the Standard Model of particle physics, photons are responsible for producing allelectric and magnetic fields, and are themselves the product of requiring that physical laws have a certainsymmetry at every point in spacetime. The intrinsic properties of photons — such as charge, mass and spin— are determined by the properties of this gauge symmetry.
The concept of photons is applied to many areas such as photochemistry, high-resolution microscopy, andmeasurements of molecular distances. Recently, photons have been studied as elements of quantum computers and for sophisticated applications in optical communication such as quantum cryptography.
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