E = h f
where h is Planck’s constant and equal to 6.63 × 10-34 Joules seconds in SI units. Einstein postulated the existence of the photon to explain the “photoelectric effect” for which he obtained the Nobel prize in physics. In the photoelectric effect individual photons can liberate electrons and stimulate a current, demonstrating the particle-like nature of light.
Curiously, although the apparent frequency of a photon can be changed by the relative motion of the observer, the velocity is always identically c, the speed of light. This is the foundation of Einstein’s special theory of relativity.