Photons :

Photons, also called light quantum, are minute energy packets of electromagnetic radiation. The concept originated in Einstein's explanation of the photoelectric effect, in which he proposed the existence of discrete energy packets during the transmission of light. The concept came into general use after the U.S. physicist Arthur H. Compton demonstrated (1923) the corpuscular nature of X-rays. The term photon (from Greek phos, photos, "light"), however, was not used until 1926. The energy of a photon depends on radiation frequency; there are photons of all energies from high-energy gamma- and X-rays, through visible light, to low-energy infrared and radio waves. All photons travel at the speed of light. Considered among the subatomic particles, photons are bosons, having no electric charge or rest mass; they are field particles that are thought to be the carriers of the electromagnetic field.

Excerpt from the Encyclopedia Britannica without permission.