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Kirchhoff, Gustav Robert

spectrum gas emission line

[keerkh hohf] (1824–87) German physicist: pioneer of spectroscopy; devised theory of electrical networks.

Basic components of a prism spectrometer. Light from the source passes through an adjustable slit, and the collimator lens forms a parallel beam. This is refracted and dispersed by a prism and the resulting spectrum is observed through a telescope fitted with cross-hairs and mounted to rotate horizontally, so that line positions in the emission spectrum can be measured.

Kirchhoff was educated at the University of Königsberg (later Kaliningrad, USSR) and spent his professional life at the universities of Breslau, Berlin and Heidelberg. An early accident made him a wheelchair user but did not alter his cheerfulness.

Kirchhoff was still a student when in 1845 he made his first important contribution to physics, formulating Kirchhoff’s Laws, which enable the current and potential at any point in a network of conductors to be determined. The two laws are extensions of Law, and state that (1) the sums of the currents in a network must be zero at circuit junctions, SI = 0, and (2) SIR = SV when applied to a closed loop in the network. Kirchhoff’s other contributions to the study of electricity include demonstrating that oscillating current in a conductor of zero resistance propagates at the speed of light, and the unification of static and current electricity.

Kirchhoff was a lifelong friend and collaborator of the chemist and it was with him that much of his work on spectroscopy was done. They established spectroscopy as an analytical technique, using the nearly colourless flame of the Bunsen gas burner and a prism system designed by Kirchhoff. They saw that a continuous spectrum is produced by a glowing solid, liquid, or a gas under high pressure. An emission-line spectrum is given by a glowing gas under low pressure. An absorption-line spectrum is shown when a cooler gas is placed between a continuous source and the observer. In their hands, the spectrometer joined the telescope and microscope as a dominant scientific instrument.

In 1860 they demonstrated that when metal compounds are heated in a flame they emit spectral lines that are characteristic of the metal concerned, a fact which led Bunsen to discover the elements caesium and rubidium shortly afterward. Kirchhoff had discovered in 1859 that the dark spectral lines in the Sun’s rays were intensified when sunlight passed through the burner flame containing certain salts, leading him to the realization that they were absorption lines corresponding to elements found in the Sun’s atmosphere. He also showed that the ratio of the emission and absorption powers of radiation of a given wavelength from all bodies is the same at the same temperature (Kirchhoff’s Law of Emission), from which he later developed the concept of the black body. The study of black body radiation was the key in the development of quantum theory.

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