define photon
A laser emits photons. Photons (from Greek ÏÏÏ, meaning light), in many atomic models in physics, are particles which transmit light. In other words, light is carried over space by photons. Photon is an elementary particle that is its own antiparticle. In quantum mechanics each photon has a characteristic quantum of energy. Photons have a rest mass of 0 (zero). However, Einstein's theory of relativity says that they do have a certain amount of momentum.