photon /fō′ton/ [Gk, phos, light] , the smallest quantity of electromagnetic energy. It has no mass and no charge but travels at the speed of light. Photons may occur in the form of x-rays, gamma rays, or quanta of light. The energy (E) of a photon is expressed as the product of its frequency (v) and Planck’s constant (h), as in the equation E = hv. X-ray photons occur in frequencies of 1018 to 1021 Hz and energies that range upward from 1 KeV.
