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A photon is a particle of light.

In classical physics light is considered to be a wave. One of the proofs for this is that light experiences interference and diffraction, which would not be displayed if light were a classical particle. Furthermore, James Clerk Maxwell theoretical prediction of electromagnetic waves, together with Heinrich Hertz experiments led to conclude that light was an electromagnetic wave.

However, a phenomenon called the photoelectric effect led to a different conclusion. The photoelectric effect consist on a metal ejecting electrons when a beam of light insides upon it. This by itself is in no contradiction to the wave theory of light, but certain peculiarities of it were. For example, no electrons were emitted unless the frequency of light was greater than a threshold frequency, regardless of how intense the light beam was. Also, the energy of the emitted electrons was dependent only on the light frequency, not in its intensity. Since the energy of a wave depends on its amplitude, which in turn is related to its intensity, this was in contradiction with the wave theory of light.

Albert Einstein solved this problem arguing that light is really a particle, which was called a photon, each of which carry an energy given by h*f, where h is Planck's constant and f is the frequency of the light. More intense light involves more photons, but the energy of each photon individually depends only on its frequency. This explained why there is a threshold frequency before a metal emits electrons: A single photon must be energetic enough to knock out an electron, and so increasing the intensity (sending more photons) will not solve the problem. Only increasing the frequency (increasing the energy per photon) will do.

In standard interpretations of quantum mechanics, light behaves both as a wave and as a particle. The existence of photons is now well accepted by the physics community, and its implication has gone well beyond the photoelectric effect.

Photons are bosons, that is, particles that have integer spin. In quantum field theory, photons are the mediators of the electromagnetic force.

See also