E=hv is used to calculate the energy of electromagnetic radiation, and is known as the Planck-Einstein relation. "E" is the energy of electromagnetic radiation, "h" is the Planck constant and "v" is the frequency of electromagnetic radiation.
In 1900, Planck introduced the hypothesis that energy is emitted in quanta, instead of a continuous emission. He proposed the equation E=hv, stating that a quantum of energy is related to the frequency. He claimed that the processes of absorption and emission of radiation had nothing to do with the physical reality of the radiation itself. In 1905, Einstein reinterpreted Planck's hypothesis and used it to explain the photoelectric effect.