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Light Waves and Their Uses
time, a considerable broadening out of the two separate elements.
This same experiment was tried with other substances, especially with cadmium, and it was found that almost identical results were obtained with cadmium light as with sodium. It was therefore inferred that the observations announced by Zeeman were, at any rate, incomplete, and it was thought that possibly the instruments at his command were not sufficiently powerful to show the phenomena of the doubling. Shortly after this experiment was published another announcement was made by Zeeman. In this he states that there is not simply a broadening of the lines, but a separation of them into three components, and, what was very much more interesting, that these three components are polarized in directions at right angles with each other: the middle line polarized in one plane and the two outer lines in another.
To make the meaning of this clear, we shall have to make a brief digression into the subject of the polarization of light. It will be remembered that in one of the first illustrations of wave motion light waves were compared with the waves along a cord, and it was stated that the vibrations which caused the phenomena of light are known to be vibrations of this character rather than of the character of sound waves. The sound waves consist of vibrations in the direction of the propagation of the sound itself. The motion of the particles in the light waves are at right angles to their direction of propagation. These transverse vibrations, as they are called, may be vertical or horizontal, or they may be diagonal, or they may move in a curved path, for instance in circles or ellipses.
In the case of ordinary light the vibrations are so mixed up together in all possible planes that it is impossible to separate any one particular vibration from the rest without
Action of Magnetism on Light Waves 111
special devices, and such devices are termed “ polarizers.” They may be likened very roughly to a grating the apertures of which determine the plane of vibration. Through such a grating we can transmit vibrations along a cord only in the plane of the apertures. A vibration at right angles to this plane will not travel along the cord beyond the grating. The corresponding light phenomena may be illustrated by attempting to pass a beam of light which has been polarized through a medium which acts toward the light waves as does the grating toward the waves on the cord. It is found that crystals act as such media. Thus a plate of tourmaline possesses this property. For, as is well known, if two plates of tourmaline be placed so that their optical axes are parallel with each other, almost as much light will pass through the two as through either one alone. But if the axes are set at right angles to each other by turning one of them through 90°, the light is entirely cut off. Turning again through 90°, the light again appears, etc. In the case of the tourmaline the vibrations which have passed through one plate are all in one plane.
There is another important case in which the light is said to be polarized, namely, when the motion of the particles is circular. We may have two such circular vibrations — one in which the motion is in the direction of the hands of a watch, called right-handed, and the other in which the motion is in the direction opposite to that of the hands of a watch, and which is therefore called left-handed. We may consider that each one of these vibrations is compounded of two plane vibrations of equal intensity, in one of which the motion is horizontal and in the other vertical, and which differ from one another in phase, this difference being one-fourth of a period for the left-handed and three-fourths of a period for the right-handed. If we add together two such circular vibrations of equal intensity, their horizontal components would exactly