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The '''Planck postulate''' (or '''Planck's postulate'''), one of the fundamental principles of [[quantum mechanics]], is the [[postulate]] that the energy of [[oscillator]]s in a [[black body]] is quantized, and is given by
:<math>E=nh\nu\,</math>,
where ''<math>n</math>'' is an integer (1, 2, 3, ...), ''<math>h</math>'' is [[Planck's constant]], and ''<math>\nu</math>'' (the Greek letter ''nu'', not the Latin letter ''v'') is the frequency of the oscillator.

The Planck Postulate was introduced by [[Max Planck]] in his derivation of [[Planck's law|his law of black body radiation]] in 1900.  This assumption allowed Planck to derive a formula for the entire [[spectrum]] of the radiation emitted by a [[black body]].  Planck was unable to justify this assumption based on [[classical physics]]; he considered quantization as being purely a mathematical trick, rather than (as we now know) a fundamental change in our understanding of the world.<ref name="Kragh">{{citation | first = Helge | last = Kragh | url = http://physicsworld.com/cws/article/print/373 | title =  Max Planck: the reluctant revolutionary | publisher = PhysicsWorld.com | date = 1 December 2000}}</ref> In other words, Planck then contemplated [[Virtual state (physics)|virtual]] oscillators.

In 1905 in one of his three most important papers, [[Albert Einstein]] adapted the Planck postulate to explain the [[photoelectric effect]], but Einstein proposed that the energy of photons themselves was quantized, and that quantization was not merely a feature of microscopic oscillators.  Planck's postulate was further applied to understanding the [[Compton effect]], and was applied by [[Niels Bohr]] to explain the [[emission spectrum]] of the [[hydrogen]] [[atom]] and derive the correct value of the [[Rydberg constant]].

== Notes ==
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== References ==
* Tipler, Paul A. (1978). ''Modern Physics'', Worth Publishers, Inc.
*[http://scienceworld.wolfram.com/physics/PlanckPostulate.html Planck Postulate] — from ''Eric Weisstein's World of Physics''

[[Category:Foundational quantum physics]]