The Planck postulate (or Planck's postulate), one of the fundamental principles of
quantum mechanics
Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistry, ...
, is the
postulate that the energy of
oscillator
Oscillation is the repetitive or periodic variation, typically in time, of some measure about a central value (often a point of equilibrium) or between two or more different states. Familiar examples of oscillation include a swinging pendulum ...
s in a
black body is quantized, and is given by
:
,
where ''
'' is an integer (1, 2, 3, ...), ''
'' is
Planck's constant, and ''
'' (the Greek letter ''nu'', not the Latin letter ''v'') is the frequency of the oscillator.
The postulate was introduced by
Max Planck in his derivation of
his law of black body radiation in 1900. This assumption allowed Planck to derive a formula for the entire
spectrum
A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum. The word was first used scientifically in optics to describe the rainbow of colors i ...
of the radiation emitted by a black body. Planck was unable to justify this assumption based on
classical physics
Classical physics is a group of physics theories that predate modern, more complete, or more widely applicable theories. If a currently accepted theory is considered to be modern, and its introduction represented a major paradigm shift, then the ...
; he considered quantization as being purely a mathematical trick, rather than (as is now known) a fundamental change in the understanding of the world.
In other words, Planck then contemplated
virtual oscillators.
In 1905,
Albert Einstein
Albert Einstein ( ; ; 14 March 1879 – 18 April 1955) was a German-born theoretical physicist, widely acknowledged to be one of the greatest and most influential physicists of all time. Einstein is best known for developing the theory ...
adapted the Planck postulate to explain the
photoelectric effect, but Einstein proposed that the energy of photons themselves was quantized (with photon energy given by the
Planck–Einstein relation
The Planck relationFrench & Taylor (1978), pp. 24, 55.Cohen-Tannoudji, Diu & Laloë (1973/1977), pp. 10–11. (referred to as Planck's energy–frequency relation,Schwinger (2001), p. 203. the Planck relation, Planck equation, and Planck formula, ...
), and that quantization was not merely a feature of microscopic oscillators. Planck's postulate was further applied to understanding the
Compton effect
Compton scattering, discovered by Arthur Holly Compton, is the scattering of a high frequency photon after an interaction with a charged particle, usually an electron. If it results in a decrease in energy (increase in wavelength) of the photon ...
, and was applied by
Niels Bohr to explain the
emission spectrum
The emission spectrum of a chemical element or chemical compound is the spectrum of frequencies of electromagnetic radiation emitted due to an electron making a atomic electron transition, transition from a high energy state to a lower energy st ...
of the
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...
atom
Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons.
Every solid, liquid, gas, and ...
and derive the correct value of the
Rydberg constant.
Notes
{{reflist
References
* Tipler, Paul A. (1978). ''Modern Physics''. Worth Publishers, Inc.
Planck Postulate€”from ''Eric Weisstein's World of Physics''
Foundational quantum physics
Max Planck