The Planck–Einstein relation is also referred to as the Einstein relation, Planck's energy–frequency relation, the Planck relation, and the Planck equation. Also the eponym 'Planck formula' belongs on this list, but also often refers instead to Planck's law 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) is proportional to its frequency (ν).
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.
Light can be characterized using several spectral quantities, such as frequency (), wavelength (), wavenumber () and their angular equivalents (angular frequency , angular wavelength , and angular wavenumber ). 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 would also apply to them, and postulated that particles would have a wavelength equal to . 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 is the momentum vector, and 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 () between the two energy levels involved in the transition:
This is a direct consequence of the Planck–Einstein relation.
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