Radiation Phenomena in the Magnetic Field

Abstract

IN the spring of 1897 the scientific world became indebted to Dr. Zeeman for the observation that when a source of light is placed in a strong magnetic field the spectral lines of the light emitted by that source suffer marked modification. The general type, or characteristic type, of this modification is that when the slit of the spectroscope views the sources of light across the lines of magnetic force, each spectral line becomes a triplet, of which the middle line has the same wave-length as the original line; whereas the side lines of the triplet have wave-lengths, respectively, a little longer and a little shorter than that of the unmodified line, the difference of wave-length being proportional to the strength of the magnetic field. Further, the central line has its vibrations parallel to the lines of force, whereas the side lines of the triplet have their vibrations perpendicular to the lines of force. Thus, if the axis of the magnetic field is horizontal, so that the lines of force are horizontal, and if the slit of the spectroscope looks horizontally across the lines of force, then in the central constituent of the triplet the vibrations are horizontal, while in the side lines the vibrations are vertical. Thus the central line is plane polarised, and the side lines are also planet polarised, but in a perpendicular plane. This is the typical phenomenon when the light is viewed across the lines of force. When the light is viewed along the lines of force—that is, through axial holes pierced in the pole-pieces of the electromagnet, the modification is different. In this case, instead of a triplet with plane polarised constituents, we are presented with a doublet, having circularly polarised constituents. That is, each spectral line is broken up into two lines of slightly different wave-length; one constituent being circularly polarised in one sense, and the other in the opposite sense. As before, the difference of wave-length, and therefore the separation of the constituents of these doublets in the spectroscope, is proportional to the strength of the magnetic field for each line, but differs in amount for the different spectral lines.