Planck's relation

The Planck–Einstein relation is also referred to as the Einstein relation, Planck's energy–frequency relation, the Planck relation, and the Planck equation. Also the eponym 'Planck formula' belongs on this list, but also often refers instead to Planck's law These various eponyms are far from standard; they are used only sporadically, neither regularly nor very widely. They refer to a formula integral to quantum mechanics, which states that the energy of a photon, , is proportional to its frequency, :

The constant of proportionality, , is known as the Planck constant. Several equivalent forms of the relation exist.