blackbodies

A blackbody is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of wavelength or angle of incidence. Because it is a perfect absorber, it also emits radiation with an emissivity of 1 at all wavelengths when in thermal equilibrium with its surroundings.

In thermodynamic equilibrium, the radiation emitted by a blackbody is determined solely by its temperature. The spectral radiance is described by Planck's law: B(λ,T) = (2 h c^2)/(λ^5) · 1/[exp(h c/(λ k_B T)) − 1], where h is Planck's constant, c the speed of light, and k_B Boltzmann's constant. A related quantity is the total emitted power per unit area, given by the Stefan-Boltzmann law: j* = σ T^4, with σ ≈ 5.670374×10^−8 W m^−2 K^−4.

Wien's displacement law provides that the wavelength at which emission peaks is inversely proportional to temperature: λ_max T ≈ 2.8978×10^−3 m K.

Real objects do not satisfy the ideal conditions, but many can be approximated as blackbodies. A perfect blackbody is an abstraction; real surfaces are grey bodies with emissivity ε < 1 that may vary with wavelength and angle. The ideal model is often approached by a cavity with a small aperture whose walls are highly absorbing.

Blackbodies have broad applications in science and engineering. They serve as calibration standards for radiative thermometers and optical detectors and provide a reference spectrum in astrophysics. The concept also underpins the study of stellar atmospheres and the cosmic microwave background.

Historically, the concept arose from Kirchhoff's law of thermal radiation and was resolved with Planck's quantum hypothesis, marking a foundation of quantum theory.