Kirchhoff's Law

Kirchhoff's Law (Kirchhoff) determines the relationship between the radiation of a heated absolutely black body and the thermal radiation of any other body at the same temperature. Let the emissive power of the black body be S_t, the emissive power of another heated body s_t, and the absorptive power of the same body a_t. Then according to K.'s law s_t = a_tS_t.

(1) K.'s law is derived for the case of purely thermal equilibrium radiation from the principle of conservation of energy and is applicable to both total radiation and any spectral region. The absorptive power a is related to the reflective power R by the relation R = 1 - a. Hence (1) can also be written in the following form: s_t = (1 - R)S_t.

(2) Thus, knowing the reflective power of the surface of a given heated body, one can determine its radiation according to the laws of the black body. The laws of radiation of an absolutely black body are known with complete accuracy; therefore, by means of K.'s law, the problem of determining the radiation of any heated body is reduced to determining the reflective or absorptive power of its surface. Kirchhoff's law of branching of electric current - see Electric current.