photon

Photon is the quantum of the electromagnetic field and the fundamental particle of light. In quantum electrodynamics, the photon is a massless, spin-1 gauge boson that mediates electromagnetic interactions. It travels at the speed of light in vacuum and has zero rest mass. Its energy and momentum are related to its frequency by E = hf and p = hf/c, where h is Planck's constant and c is the speed of light. Photons exhibit both wave-like and particle-like properties, with the wave description accounting for frequency and polarization and the particle aspect explaining emission, absorption, and blackbody radiation.

Photons can be created or destroyed in processes such as atomic transitions, spontaneous and stimulated emission, bremsstrahlung, and Compton scattering. They interact with charged matter through absorption and scattering, and their polarization describes the orientation of the electromagnetic field. For massless photons, the helicity states reduce to two polarization possibilities. In quantum optics, photons are treated as quanta of the quantized electromagnetic field and obey Bose-Einstein statistics.

Speed of photons is the speed of light in vacuum, c, but their effective speed can be reduced in media with refractive index. In materials they can be absorbed and re-emitted, refracted, or reflected. Technological applications include optical communication, lasers, imaging, solar energy conversion, photodetection, and various aspects of quantum information science such as entanglement and quantum cryptography.

Historically, the photon concept developed from Planck's work on blackbody radiation and Einstein's explanation of the photoelectric effect in 1905, leading to the development of quantum electrodynamics. In modern theory, photons are the gauge bosons of the U(1) electromagnetic symmetry, mediating the electromagnetic force between charged particles.