# Planck–Einstein relation

The Planck–Einstein relation[1][2][3] is also referred to as the Einstein relation,[1][4][5] Planck's energy–frequency relation,[6] the Planck relation,[7] and the Planck equation.[8] Also the eponym Planck formula[9] belongs on this list, but also often refers to Planck's law instead.[10][11] 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, E, known as photon energy, is proportional to its frequency, ν:

${\displaystyle E=h\nu }$

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

The relation accounts for quantized nature of light, and plays a key role in understanding phenomena such as the photoelectric effect, and Planck's law of black body radiation. See also the Planck postulate.

1. ^ a b French & Taylor (1978), pp. 24, 55.
2. ^ Cohen-Tannoudji, Diu & Laloë (1973/1977), pp. 10–11.
3. ^ Kalckar 1985, p. 39.
4. ^ Messiah (1958/1961), p. 72.
5. ^ Weinberg (1995), p. 3.
6. ^ Schwinger (2001), p. 203.
7. ^ Landsberg (1978), p. 199.
8. ^ Landé (1951), p. 12.
9. ^ Griffiths, D.J. (1995), pp. 143, 216.
10. ^ Griffiths, D.J. (1995), pp. 217, 312.
11. ^ Weinberg (2013), pp. 24, 28, 31.