Photon

Photon, pronounced FOH tahn, is the elementary particle that makes up light and all other forms of electromagnetic radiation. Like all particles, photons have properties of waves, including frequency and wavelength. But photons have no mass and no electric charge. The speed at which photons travel in a vacuum is the speed of light. The exchange of photons between electrically charged particles transmits the electromagnetic force, one of the four fundamental forces.

In 1900, the German physicist Max Planck showed that the energy of a photon (then called a radiation quantum) is proportional to the frequency of its light. The German-born physicist Albert Einstein carried Planck's discovery further. In 1905, Einstein used the idea of photons to explain the ability of light to knock electrons out of atoms--a phenomenon known as the photoelectric effect. His predictions about photons, which were later confirmed by experiment, contributed much to the development of the quantum theory.

Contributor: Joel R. Primack, Ph.D., Prof. of Physics, Univ. of California, Santa Cruz.

See also Light; Quantum Mechanics; Radiation.

Photon

Master Index