Planck's law

Planck's law gives the spectral radiance of electromagnetic radiation emitted by a body in thermal equilibrium as a function of wavelength or frequency and temperature.

Black-body radiation: Electromagnetic radiation emitted by an ideal absorber in thermal equilibrium, whose spectrum depends only on temperature. Planck's law specifies the spectrum of this idealized thermal radiation.
Max Planck: A German theoretical physicist whose work on black-body radiation introduced the quantum of action. Planck derived the radiation formula while seeking agreement with measured spectra.
Color temperature: A temperature scale describing the color of a light source by comparison with black-body radiation. Planck spectra define the reference colors used to assign this temperature.
Emissivity: A measure of how effectively a real surface emits thermal radiation relative to a black body at the same temperature. Real materials require emissivity corrections because Planck's law gives the ideal black-body spectrum.
Planck constant: A fundamental constant setting the scale of quantum effects and relating a photon's energy to its frequency. It sets the energy spacing that shapes Planck's spectrum.
Ultraviolet catastrophe: The classical prediction that a black body emits unbounded energy at short wavelengths. Planck's law resolved this failure of classical predictions.
Thermal radiation: Electromagnetic radiation emitted by matter because of its temperature. Planck's law quantitatively describes the ideal equilibrium case of thermal radiation.
Gray body: An idealized emitter whose emissivity is constant across wavelength but less than that of a black body. It modifies the ideal spectrum by a wavelength-independent emission factor.
Photon: The quantum of the electromagnetic field, carrying energy proportional to its frequency. Planck's quantized energy elements correspond to photons in the quantum account of radiation.
Kirchhoff's law of thermal radiation: A principle stating that, at thermal equilibrium, a body's emission and absorption properties are linked at each wavelength. It framed the universal radiation problem Planck's formula addressed.

Linked from 43 pages