The discovery at the turn of the nineteenth century that the light from the Sun dispersed through a spectroscope produced not only a continuous spectrum, as Newton had shown in 1666. But one crossed by dark lines – the Fraunhofer lines as they came to be known – provoked much concern among astronomers, physicists and chemists as to their nature. Both continuous and bright-line spectra had been observed previously, but the interruption of a continuous spectrum by numerous dark lines was a new phenomenon
To add to the mystery of these lines, some were found to coincide in position with the bright lines obtained from normal laboratory spectra of the elements. In 1840-60, a number of experiments were undertaken, primarily by scientists in England, France and Germany, that came very close to the final interpretation. How¬ever, they did not arrive at the general explanation of spectra; this was provided by Gustav Kirchhoff.
Kirchhoff had conducted experiments on the spectra of flames and metallic vapours, with particular emphasis on a certain line in the solar spectrum known, according to Fraunhofer’s classification, as the sodium D- line. He noted that by superimposing a sodium Maine on a solar spectrum, the dark solar D-line changed into a bright line. Similar experiments were conducted with other elements, and in 1859 Kirchhoff was able to announce the two basic laws of spectroscopy which are named after him: firstly, that incandescent solids or liquids produce a continuous spectrum, while gaseous substances produce either bright lines or bands with each chemical element producing a characteristic spectrum; and secondly that each element is capable of absorbing the same radiation that it emits. These rules formed the basis of early spectroscopy, and immediately opened the doors to a new field of investigation – the science of astrophysics.