Kirchhoff's Radiation Law

a law stating that the ratio of emmissive power ε(λ,T) of bodies to their absorptivity α(λ,T) is independent of the nature of the radiating body. This ratio is equal to the emissive power of the black body ε₀(λ,T) (because its absorptivity is equal to 1) and depends on the radiation wavelength λ and on the absolute temperature T:

The function ε₀(λ,T) is given in explicit form by Planck’s radiation formula.

Kirchhoff’s radiation law is one of the fundamental laws of thermal radiation and does not apply to other types of radiation. The law was established by G. R. Kirchhoff in 1859 on the basis of the second law of thermodynamics and was subsequently confirmed experimentally. According to Kirchhoff’s radiation law a body that, at a given temperature, exhibits a stronger absorptivity must also exhibit a more intensive emission. For instance, if a platinum plate partially covered with platinum black is heated to incandescence, the blackened end will glow much brighter than the light end.