Planck's law, discovered by Max Planck, is the basis of the quantum theory of electromagnetism.
The law states that the energy associated with electromagnetic radiation comes in indivisible packets called quanta, each of which is associated with a single photon. The size (E) of a quantum depends on the frequency (f) of the radiation, according to the formula
- E = hf,
where h is Planck's constant.
Using SI units, energy is measured in joules, frequency is measured in hertz, and Planck's constant has the unit joule seconds.