The Planck relation[1][2][3] (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 law[8][9]) is a fundamental equation in quantum mechanics which states that the energy E of a photon, known as photon energy, is proportional to its frequency ν:
Several equivalent forms of the relation exist, including in terms of angular frequency ω:
Written using the symbol f for frequency, the relation is
The relation accounts for the quantized nature of light and plays a key role in understanding phenomena such as the photoelectric effect and black-body radiation (where the related Planck postulate can be used to derive Planck's law).
Light can be characterized using several spectral quantities, such as frequency ν, wavelength λ, wavenumber
so the Planck relation can take the following "standard" forms:
The standard forms make use of the Planck constant h. The angular forms make use of the reduced Planck constant ħ = h/2π.
The de Broglie relation,[10][11][12] also known as de Broglie's momentum–wavelength relation,[4] generalizes the Planck relation to matter waves.
Louis de Broglie argued that if particles had a wave nature, the relation E = hν would also apply to them, and postulated that particles would have a wavelength equal to λ = h/p.
Combining de Broglie's postulate with the Planck–Einstein relation leads to
The de Broglie relation is also often encountered in vector form
where p is the momentum vector, and k is the angular wave vector.
Bohr's frequency condition[13] states that the frequency of a photon absorbed or emitted during an electronic transition is related to the energy difference (ΔE) between the two energy levels involved in the transition:[14]
This is a direct consequence of the Planck–Einstein relation.