# Planck-Einstein relation

(Redirected from Planck's relation)

The Planck-Einstein relation is the formula

$E\,=\,h\nu\,=\,\hbar \omega,$

where

E is the energy of a photon
ν the photon's frequency
ω the photon's angular frequency
h is Planck's constant
ħ reduced Planck's constant, where ħ = h/2π.[1][2]

The formula is also referred to as the Einstein relation,[1][3][4] and as Planck's energy–frequency relation,[5] and as the Planck relation.[6]

## Alternative usages

The term Planck–Einstein relation also can refer to the closely related formula

$\mathbf p\,=\,\hbar \mathbf k\,,$

where p denotes the momentum of the photon and k its angular wavenumber.[2]

The term Einstein relation can also refer to the formula

$E\,=\,mc^2,$

where E denotes the energy of a particle and m its mass; c denotes the speed of light.[7]

## Related formula

The formula E = is related to Bohr's frequency condition

$E'\,-\,E''\,=\,h\nu,$

where E and E′′ denote the source and destination energy levels of an atom that emits the photon.[8]

## References

1. ^ a b French, A.P., Taylor, E.F. (1978), pp. 24, 55.
2. ^ a b Cohen-Tannoudji, C., Diu, B., Laloë, F. (1973/1977), pp. 10–11.
3. ^ Messiah, A. (1958/1961), p. 72.
4. ^ Weinberg, S. (1995), p. 3.
5. ^ Schwinger, J. (2001), p. 203.
6. ^ Landsberg, P.T. (1978), p. 199.
7. ^ Schwinger, J. (2001), p. 22.
8. ^ van der Waerden, B.L. (1967), p. 5.