The massless particle that, according to the quantum theory of radiation, carries the smallest discrete amount of electromagnetic energy hν, where h is Planck's constant and ν is the frequency of the associated electromagnetic wave. Although the photon has no mass it does have (linear) momentum hν/c, where c is the free- space speed of light, and intrinsic angular momentum (spin) h/2π, as evidenced by radiation pressure and radiation torque.