The Planck–Einstein relation (referred to by different authors as the Einstein relation, Planck's energy–frequency relation, the Planck relation, Planck equation, and Planck formula, though the latter might also refer to Planck's law) is a fundamental equation in quantum mechanics which states that the energy of a photon, E, known as photon energy, is proportional to its frequency, ν:
where . 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 , and their angular equivalents (angular frequency ω, angular wavelength y, and angular wavenumber k). These quantities are related through
so the Planck relation can take the following 'standard' forms
as well as the following 'angular' forms,
de Broglie relation
The de Broglie relation, also known as the de Broglie's momentum–wavelength relation, 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/. Combining de Broglie's postulate with the Planck–Einstein relation leads to
The de Broglie's 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
Bohr's frequency condition 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:
This is a direct consequence of the Planck–Einstein relation.
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