The Planck relation123 (referred to as Planck's energy–frequency relation,4 the Planck–Einstein relation,5 Planck equation,6 and Planck formula,7 though the latter might also refer to Planck's law89) is a fundamental equation in quantum mechanics which states that the photon energy E is proportional to the photon frequency ν (or f):
The constant of proportionality, h, is known as the Planck constant. Several equivalent forms of the relation exist, including in terms of angular frequency ω:
where the reduced Planck constant is .
The relation accounts for the quantized nature of light and plays a key role in understanding phenomena such as the and (where the related can be used to derive ).