Kirchhoff's radiation law states that at thermal equilibrium, the emissivity of a body equals its absorptivity.In 1859, Gustav Kirchhoff, together with chemist Robert Bunsen, lit a burner and noticed: sodium vapor gave a bright yellow line in the flame; and if white light was passed through it, a dark band appeared in the same place in the spectrum. Thus spectroscopy was born. Kirchhoff generalized: for any body in thermal equilibrium, the ratio of emissive power E(λ,T) to absorptivity A(λ,T) is a universal function depending only on wavelength and temperature, not the material. The function f(λ,T) is the famous blackbody radiation later described by Planck. A simple consequence: a good absorber is always a good emitter. Thanks to Kirchhoff's law, astronomers deciphered the chemistry of the Sun and distant stars.
Gustav Kirchhoff, professor at Heidelberg University, in 1859–1862 theoretically established the inseparable connection between absorption and emission of thermal radiation. Together with Robert Bunsen, he created the method of spectral analysis and demonstrated that each chemical element gives a unique set of spectral lines. Kirchhoff's law became a cornerstone of astrophysics and the physics of thermal radiation, pointing the way to the blackbody function, later derived by Max Planck. The idea of the universality of blackbody radiation stimulated the development of quantum theory.
The law strictly holds only under conditions of thermodynamic equilibrium: the temperature of the emitter and the radiation field must be the same. In real laboratory or astronomical conditions, this approximation is often satisfied. The law underpins methods of pyrometry, infrared thermography, and the design of spacecraft thermal protection. Furthermore, it allows the chemical composition of gases and plasmas to be deduced from absorption spectra.
All pyrometers (non-contact thermometers) operate on Kirchhoff's law: by measuring the infrared radiation from a surface, we calculate its temperature based on its known emissivity. In astrophysics, stellar spectra with dark absorption lines directly reveal the chemical composition of their atmospheres.
It is precisely because of Kirchhoff's law that infrared motion sensors see people but not glass: glass is transparent to visible light and barely emits in the IR, while a person emits as an almost perfect blackbody.
