Anyone who has ever used a prism to split a beam of light into its component colors has, to a small degree, practiced spectroscopy. Widely used in the study of radiation, spectroscopy is based upon the principle of separation of light according to its frequencies, or wavelengths. In our prism example, the many frequencies within a beam of white light are dispersed, revealing a spectrum. By examining which colors within that spectrum are brighter or dimmer, a scientist can obtain information about the source of the white light.
Although many scientists (including Isaac Newton) had experimented with prisms, it was the physicist Joseph von Fraunhofer (1787-1826) who first applied spectroscopy to analytical science. While examining some rather outmoded lenses, he discovered that the spectrum of the Sun 's light, until this time thought to be continuous, was actually interrupted by hundreds of dark lines. His contemporaries claimed that the lines were caused by imperfections in the lenses, but the intuitive Fraunhofer knew that they were characteristic of the light itself. Unfortunately, in 1814 the technology did not exist to examine the phenomenon any further.
It was not until 1859 that Robert Bunsen and Gustav Kirchhoff designed a spectrometer that could record the spectra of light. Bunsen had been interested in the behavior of elements when they were superheated; upon teaming with Kirchhoff, they found that each element emits a particular color of light when heated or burned. (Bunsen invented the laboratory burner bearing his name for this purpose). When the elements' lights were examined through a prism it was found that each emission spectra was unique to its element, like a spectroscopic fingerprint. Using their new device, Bunsen and Kirchhoff catalogued the spectra of all the known elements, as well as discovered two new elements--cesium and rubidium.
Its importance to analytical chemistry evident, spectroscopy was seized upon next by astronomers who recognized that the spectra of distant stars could be observed through a spectroscope to determine their chemical makeup. White light from the hot surface of a star passes through it's somewhat cooler atmosphere where some of the light is absorbed by the atoms in the atmosphere. The absorption takes place at those frequencies which are characteristic of the gases in the stellar atmosphere and are called absorption spectra. There are a few examples in which emission lines have also been identified in stellar spectra. These are caused by some form of excitation of the atoms in the stellar atmosphere The first scientist to make a systematic spectroscopic study of the heavens was the Italian Pietro Angelo Secchi (1818-1878), who examined the spectra of nearly four thousand stars during the late 1860s.
Spectroscopy is not confined to the realms of visible light. In 1912 the father-and-son team of William Henry Bragg (1862-1942) and William Lawrence Bragg (1890-1971) developed a device that could examine the spectral patterns of X-rays using a diffraction grating rather than a prism. Diffraction gratings employed close-set grooves to disperse the frequencies within light. Since X-rays have much shorter wavelengths than do light rays, it was important to find a diffraction grating whose grooves were set very close together, much closer than any machine could cut them. The Braggs found the ideal natural diffraction grating in a crystal, whose layered atomic structure diffused the X-rays perfectly.
The Braggs' effort gave the world a greater understanding of how X-rays function. It also served as the cornerstone for the creation of a new science, X-ray crystallography. Since the spectral pattern of an X-ray is caused by the action of striking the many atoms within a crystal, a crystallographer can use the pattern to determine the crystal's complex atomic structure; thus, the crystallographer is able to duplicate any crystal substance. This method was used most successfully to synthesize penicillin, vitamin B12, and insulin.
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