Planck postulate

The hypothesis that matter exchanges energy with radiation in discrete amounts, each quantum equal to Planck’s constant times the radiation frequency. It was introduced to explain the spectrum of blackbody radiation.

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Planck’s law: A formula giving the spectral distribution of electromagnetic radiation emitted by a blackbody at a given temperature. It is the spectrum Planck derived by assigning oscillators discrete energy values.
Max Planck: German theoretical physicist who introduced the quantum hypothesis while studying blackbody radiation. He proposed the postulate in 1900 to derive the observed spectrum.
Photon: The quantum of the electromagnetic field, carrying energy proportional to its frequency. Einstein later treated light itself as consisting of energy quanta.
Classical electromagnetism: The theory of electric and magnetic fields described by Maxwell’s equations. Its continuous treatment of radiation alone cannot produce Planck’s thermal spectrum.
Blackbody radiation: Electromagnetic radiation emitted by an ideal absorber and emitter, with a spectrum determined by temperature. Its measured spectrum was the problem the postulate resolved.
Blackbody radiation experiments: Measurements of thermal radiation from cavity-like sources across wavelength and temperature. These spectra provided the empirical target for Planck’s derivation.
Photoelectric effect: The emission of electrons from a material after it absorbs electromagnetic radiation. Einstein used light quanta to explain its frequency-dependent behavior.
Einstein–Planck relation: The relation E = hν connecting a photon’s energy with its frequency. It shares the same energy-frequency formula but applies it to individual light quanta.
Energy quantum: A discrete amount of energy associated with a quantized physical process. The postulate sets this amount to Planck’s constant multiplied by frequency.
Boltzmann entropy formula: The relation between entropy and the number of microscopic states, commonly written S = k log W. Planck used Boltzmann’s statistical method in deriving the radiation law.