Zeeman Effect

ZEEMAN EFFECT, named after its discoverer, is the term used to describe the phenomena produced in spectroscopy (q.v.) by a magnetic field. When a substance, which emits a line spectrum, is placed in a strong magnetic field, every line is split up into several components each of which has a characteristic change of frequency and characteristic polarization and intensity. Magnetism produces many curious effects in matter (change of electrical resistance, "Hall" effect, etc.), but the Zeeman effect has an importance immensely greater than the rest because it has proved to be one of the most powerful means of discovering the nature of the forces in the atom. The first indication of a connec tion between light and magnetism was due to Faraday, who dis covered the magnetic gyration of light in 1845 ; i.e., when plane polarized light goes through transparent matter in a magnetic field the plane of polarization is rotated. With extraordinary insight he conjectured that there ought to be a corresponding effect in the emission of light, and almost the last experiment of his life was to seek for it. He failed to detect it since the technique both of spectroscopes and of magnetic fields was insufficiently developed, but modern theory entirely bears out the correctness of his con jecture. In 1896 Zeeman made a similar attempt and was suc cessful. When a source of light, such as a metallic arc, was placed between the poles of a powerful electromagnet, the lines of its spectrum were split into components, some displaced to the red and some to the blue, and each of these was polarized in a •char acteristic way.

The experimental study of the Zeeman effect calls for little comment. Even in the strongest magnetic fields available (say 30,00o gauss) the extreme components into which a line is split are never more than about i A. U. apart, and there may well be a dozen or so components between them, so that it is not only necessary to have a powerful magnet, but also a spectroscope of high resolving power. The consequent difficulties are very great,

but are not peculiar to the Zeeman effect. The polarizations of the various components are shown by the ordinary methods used for polarized light, and their intensities by means of an opaque wedge according to the usual photometric practice, and only one point calls for comment. Light passing through a diffraction grat ing has its intensity differently affected according as it is polar ized along or across the lines of the grating, and this might en tirely vitiate the comparison of the intensities of lines polarized in these two ways. The difficulty is avoided by the use of a quartz plate, which rotates the planes of polarization so that both types are at to the lines of the grating. It was only after this was done that it was found possible to obtain correct values for the intensities, with important consequences for the general theory of spectra.

The Normal Zeeman Effect.

Almost immediately after Zeeman had made his discovery, Lorentz showed how it would fit into the classical electric theory. His explanation has been superseded in the light of later knowledge, but it still furnishes a convenient description of the simpler features of the effect. It explains what is called the "normal effect," though, as so often happens with scientific nomenclature, it turns out that the normal effect is of rather rare occurrence and is not in fact the most primitive type. Lorentz supposed that the atom contains an elec tron which describes free vibrations about a centre. The restrain ing force is proportional to the distance from the centre and is the same for all directions, so that the free period is independent of the orbit of the electron. Such an electron will emit light of its own frequency and with polarization completely determined by its orbit. When there is no external magnetic field the orbits of the various atoms in the source are orientated at random, and their average effects give unpolarized light.