Kirchhoff's Radiation Law

The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

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.

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.
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Experimenting theory: The proofs of Kirchhoff's radiation law before and after Planck.