A mathematical description of the energy radiated at different wavelengths by a black body: E = hf, where E is the energy of a photon and f its frequency. It was formulated in 1900 by Max Planck (1858–1947), who realized that energy is radiated in discrete packets, which he called quanta, and it formed the basis of quantum theory. The quantum of light is a photon, the energy of which depends on its wavelength.