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.