A blackbody is an idealisation of a piece of matter that absorbs all incident radiation and is in equilibrium with radiation. Since a piece of matter in equilibrium with radiation emits the same amount of radiation it absorbs, a black body is said to be a "perfect emitter of radiation". The piece of matter is called a *black body* because a black piece of matter absorbs more radiation than matter of any other colour.

In 1900, Max Planck presented the following empirical formula for the amount of energy per volume in a cavity for the frequency range: *f* and *f*+*df*:

This formula was found to fit Observations better than we would expect of an empirical formula, and Planck had to find a fundamental justification for his formula. Although Planck considered electrons oscillating in the walls of the cavity, here we shall consider the standing waves within a cavity.