Planck postulate

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The Planck Postulate (or Planck's Postulate) was used by Max Planck in his derivation of his law of black body radiation in 1900. It is the postulate that the energy of oscillators in a black body is quantised by:

$E=nh\nu\,$ ,

where n = 1, 2, 3, ..., h is Planck's constant, and ν is the frequency.

This assumption allowed Planck to derive a formula for the entire spectrum of a black body. Prior to Planck, classical calculations based on a continuous spectrum of oscillator energies gave the Rayleigh-Jeans Law. This formula was correct at low frequencies but diverged at high frequencies, a problem called the ultraviolet catastrophe.

Planck was unable to justify this assumption based on classical physics. In 1905 in one of his three most important papers, Einstein adapted the Planck postulate to explain the photoelectric effect, but Einstein proposed that the energy of photons themselves were 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 Neils Bohr to explain the spectrum of the hydrogen atom and derive the correct value of the Rydberg constant.