Black-body radiation

Black-body radiation is electromagnetic radiation emitted by an ideal absorber in thermal equilibrium. Its spectrum depends only on the body's temperature.

Planck's law: A formula giving the spectral radiance of black-body radiation as a function of frequency or wavelength and temperature. It specifies the spectrum and introduced energy quantization to match observations.
Gustav Kirchhoff: A German physicist whose work on spectroscopy and thermal radiation established general principles of emission and absorption. He formulated the universality of thermal radiation and helped define the black-body problem.
Thermal equilibrium: A state in which interacting systems exchange energy without net changes in their macroscopic properties. Black-body radiation is the equilibrium radiation field associated with a body's temperature.
Thermal radiation: Electromagnetic radiation emitted by matter because of its temperature. Black-body radiation is the ideal limiting case of thermal radiation.
Gray body: An idealized thermal emitter whose emissivity is less than one but independent of wavelength. It relaxes the perfect-absorption assumption while retaining a simple thermal spectrum.
Stefan–Boltzmann law: A law stating that total black-body radiant power per unit area is proportional to the fourth power of absolute temperature. Integrating the spectrum yields this temperature dependence of total emitted power.
Max Planck: A German theoretical physicist who introduced energy quantization while deriving the black-body spectrum. His 1900 radiation law matched measurements and began quantum theory.
Absorptivity: The fraction of incident radiation absorbed by a material at a given wavelength and direction. A black body is defined by absorptivity equal to one for all incident radiation.
Color temperature: The temperature of a black body whose color most closely matches that of a given light source. It uses the black-body spectrum to characterize the appearance of real light sources.
Selective emitter: A thermal emitter whose emissivity varies substantially with wavelength. Its spectrum departs from the universal black-body shape because material properties matter.

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