Spectroscopy

SPECTROSCOPY (from Lat. spectrum, an appearance, and Gr. to see), has for its province the investigation of spectra, which are the appearances observed when the radiations from a luminous source are separated into their constituent parts. Such separation can be effected by the use of a prism, or by a diffraction grating, and instruments which are designed for the convenient use of these appliances are called spectroscopes.

This important branch of physics came especially into promi nence through the work of Kirchhoff and Bunsen in 1859, when it was primarily regarded as providing a new method of investigating the chemical nature of substances, and was called spectrum analy sis. A number of elements new to chemistry were soon afterwards discovered by this method, which was found to be extremely sensi tive, so that, for example, one three-millionth of a milligram of sodium could be assigned to that element with certainty. Being independent of the distance of the luminous source, the spectro scopic method was clearly applicable to the sun and other celes tial bodies, and the science of Astrophysics came into existence.

It was soon realized, however, that spectrum analysis was not so simple a matter as it first appeared. The early experiments apparently suggested that each element had its characteristic and invariable spectrum, irrespective of whether it existed in the free state or in combination with other elements. But as inquiry was extended it was found that many compounds as well as elements gave their characteristic spectra, and, further, that even an ele ment could give different spectra according to the physical con ditions under which it was excited to luminosity.

The spectrum has thus become much more than a key to chemi cal composition; it has also become a key to the physical condi tions under which the corresponding radiation is excited; and, as was clearly foreseen by some of the earlier workers, it has gone far to reveal the structure of atoms and molecules. The old name being no longer appropriate, the word "Spectroscopy" was intro duced to cover the whole range of spectroscopic investigations, apparently by Sir Arthur Schuster in a lecture delivered at the Royal Institution in 1882.

The Simple Prism.

The production of a spectrum by the use of a prism depends upon the fact that rays of light of different colours, or wave-lengths, are refracted differently on entering or emerging obliquely from one medium to another of different density. Thus, in fig. 1 the ray of light AP, consisting of a mix ture of red and violet light, will be split up into its two compo nents by the prism, the red following the path PDR, while the violet, being more refracted than the red, will follow the path PEV. A composite ray of light is accordingly dis persed, as well as refracted, in passing through a prism. If the source of light be white, the spectrum will show a suc cession of colours like those of the rainbow ; namely, red, orange, yellow, green, blue, indigo and violet. Beyond the visible violet there is a long range of spectrum called the ultra-violet, and there is similarly a region called the infra-red beyond the visible red end of the spectrum.

The essential parts of a simple prismatic spectroscope are repre sented diagrammatically in fig. 2. Light from the source under examination enters the instrument through a narrow slit at S, which is placed at the principal focus of a lens A. Parallel rays of light of mixed colour thus fall on the prism, and each component bundle of rays continues parallel in its passage through the prism and on emergence into the air beyond. The different bundles, how ever, are refracted differently, and, when they pass through the lens B are focussed at different points, so as to form a spectrum.

' The spectrum may then be ob served with a suitable eyepiece, or it may be photographed direct. The lens B and the eyepiece are ordinarily combined in the form of a telescope, which can be rotated about the centre of the prism, so that different parts of the spectrum can be observed. When the eyepiece is replaced by a camera, the instrument is called a spectrograph.