Kirchhoff’s Laws

The first clue to the origin of spectral lines was found by Kirchhoff who, in the mid-1800’s, developed the three laws of spectroscopic analysis. Gustav Kirchhoff (b. March 12, 1824, Knigsberg, Prussia [now Kaliningrad, Russia]–d. Oct. 17, 1887, Berlin, Ger.), German physicist who, with the chemist Robert Bunsen, firmly established the theory of spectrum analysis (a technique for chemical analysis by analyzing the light emitted by a heated material), which Kirchhoff applied to determine the composition of the Sun.kirchhoff

In 1847 Kirchhoff became Privatdozent (unsalaried lecturer) at the University of Berlin and three years later accepted the post of extraordinary professor of physics at the University of Breslau. In 1854 he was appointed professor of physics at the University of Heidelberg, where he joined forces with Bunsen and founded spectrum analysis. They demonstrated that every element gives off a characteristic colored light when heated to incandescence. This light, when separated by a prism, has a pattern of individual wavelengths specific for each element. Applying this new research tool, they discovered two new elements, cesium (1860) and rubidium (1861).

Kirchhoff went further to apply spectrum analysis to study the composition of the Sun. He found that when light passes through a gas, the gas absorbs those wavelengths that it would emit if heated. He used this principle to explain the numerous dark lines (Fraunhofer lines) in the Sun’s spectrum. That discovery marked the beginning of a new era in astronomy.

In 1875 Kirchhoff was appointed to the chair of mathematical physics at the University of Berlin. Most notable of his published works are Vorlesungen ber mathematische Physik (4 vol., 1876-94; “Lectures on Mathematical Physics”) and Gesammelte Abhandlungen (1882; supplement, 1891; “Collected Essays”).

There remains a problem with the spectra and the Rutherford model of the atom. Rutherford atoms can absorb or emit photons at any wavelength because the orbit of the electron can take on any distance from the nucleus. However, the spectral lines indicate that there are preferred orbits for the electrons, that they have certain discrete distances from the nuclei. This `quantization’ of electron orbits leads to the next great breakthrough in modern physics, quantum mechanics.

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