What is the energy of a photon?

A photon is a particle that has zero mass, has no electric charge (but has two states of polarization), which is constantly moving at the speed of light. When passing through a material medium, the photon energy is invariant, because the frequency is invariant. The energy of a photon depends solely on the frequency of the light.

Photons were originally described by Einstein in their interaction with electrons. Indeed, it was in his attempt to explain the photoelectric effect that Einstein put forward the hypothesis of “energy quanta” that make up light.