n.
or light quantum
Minute energy packet of electromagnetic radiation .
In 1900 Max Planck found that heat radiation is emitted and absorbed in distinct units, which he called quanta. In 1905 Albert Einstein explained the photoelectric effect , proposing the existence of discrete energy packets in light. The term photon came into use for these packets in 1926. The energies of photons range from high-energy gamma rays and X rays to low-energy infrared and radio waves, though all travel at the same speed, the speed of light. Photons have no electric charge or rest mass and are the carriers of the electromagnetic field .