Gustav Robert Kirchhoff (March 12, 1824 - October 17, 1887), physicist who is well known for his electrical rules, Kirchhoff's voltage law and Kirchhoff's current law, which are fundamental to circuit analysis in electrical engineering. He was born in Koenigsberg, Prussia (now Kaliningrad, Russia) and died in Berlin, Germany.

In his spectroscopic collaboration with Bunsen, he was a co-discoverer of caesium and rubidium.

Kirchhoff's Law of Radiation

Kirchhoff formulated the following law in 1859, followed by a proof in 1861.

The rate of emission of energy by a body is equal to the rate at which the body absorbs energy (both emission and absorption being in a given direction at a given wavelength).

In 1862 he introduced the term black body radiation. Later, he produced three empirical laws describing the spectral composition of light emitted by incandescent objects.

Kirchhoff's Radiation Laws

1. A hot solid object produces light with a continuous spectrum.
2. A hot tenuous gas produces light with spectral lines at discrete wavelengths (i.e. specific colors) which depend on the energy levels of the atoms in the gas.
3. A hot solid object surrounded by a cool tenuous gas (i.e. cooler than the hot object) produces light with an almost continuous spectrum which has gaps at discrete wavelengths depending on the energy levels of the atoms in the gas.

Reference

• Short biography (http://chem.ch.huji.ac.il/~eugeniik/history/kirchhoff.htm)