A name for the discrete particles of pure energy that represent light, or electromagnetic radiation.

Photons have an energy *E* associated with their frequency *f* of oscillation such that:

*E* = *h* *f*

where *h* is Planck’s constant and equal to 6.63 × 10^{-34} Joules seconds in SI units. Einstein postulated the existence of the photon to explain the “photoelectric effect” for which he obtained the Nobel prize in physics. In the photoelectric effect individual photons can liberate electrons and stimulate a current, demonstrating the particle-like nature of light.

Photons travel at the speed of light in a vacuum and are created by atomic transitions, thermal motions and more extreme phenomena such as matter-anti matter annihilations.

Photons can be linearly or circularly polarised, and have a momentum associated with them. Photons have a characteristic wavelength associated with them given by the wave equation.

Curiously, although the apparent frequency of a photon can be changed by the relative motion of the observer, the velocity is always identically *c*, the speed of light. This is the foundation of Einstein’s special theory of relativity.

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