Michelson A. A. Light waves and their uses (1903)

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Action of Magnetism on Light Waves 118

tide is termed an “ electron.” This hypothesis, made long before Zeeman made his discovery, was found necessary to account for the facts of electrolysis. For the decomposition of an electrolyte by an electric current is most simply explained upon the hypothesis that it contains positively and negatively charged particles, and that the positively charged atoms go toward the negative pole, and the negatively charged toward the positive pole. They then give up their electricity, and this giving up of electricity constitutes an electric current. Hence this assumption, which is useful in explaining the Zeeman effect, is nothing new. It is known, also, that the vibrations of these particles, or of their electric charges, produce the disturbance in the ether which is propagated in the form of light waves; and that the period of any light wave corresponds to the p>eriod of vibration of the electric charge which produces it.

The most general form of p)ath of such a vibrating electric charge would be an ellipse. Now, an elliptical vibration can always be resolved into a circular vibration and a plane one, so that any polarized ray may be resolved into a plane polarized ray and a circularly polarized ray. So all we need to consider are p>lane and circularly polarized rays. But wre may suppose that a plane vibration is due to two oscillations in a circle, one going in a direction opposite to that of the hands of a watch, and the other in their direction. Hence, we need consider only circular vibrations. Now, if the electric charge is moving in a circle, it can be shown that when the plane of the circle is at right angles to the direction between the two poles of the magnet, the effect of the field would bo to accelerate the motion when the rotation is, say, counter-clockwise, but to retard it when it is clockwise.

It was shown above that the position of a spectral line in the spectrum deponds on the p>eriod of the light which produces it. Hence the position of the line will be altered


Light Waves axd Their Uses

when any current is passing about the electro-magnet. When the current is passing in a certain direction, the velocity of rotation of the particles moving, say counter-clockwise, is increased. Hence the period of vibration is smaller; I. e., the number of vibrations, or the frequency, is greater. In this case there will be a shifting toward the blue end of the spectrum by an amount corresponding to the amount of the acceleration. Those particles which are rotating in an opposite direction, i. r\, clockwise, will be retarded, the frequency will be less, and the spectral lines will be shifted toward the red. These two shiftings would account, then, for the double line. It is further clear that those vibrations which occurred in planes parallel to the lines of force of the' magnetic field would be unaltered. These vibrations would then produce the middle line, which is not shifted from its position by the magnetic field.

Again, if we are viewing the light in a direction at right angles to the lines of force of the field, the vibrations of those particles which are affected by the field would have no components parallel to the field. If the particles are revolving in a plane perpendicular to the field, then, when viewed in this direction, they would appear to be moving only up and down; i. e., they would send out plane polarized light whose vibrations are vertical. These two vertical vibrations form the two outer lines of the triplet, and it can be shown that the light is plane polarized by passing it through a polarizer. Those particles which are vibrating horizontally do not have their period of vibration altered by the field. Consequently we get a single line whose position in the spectrum is not changed, and wThicli is plane polarized in a plane at right angles to that of the other two.

When this second announcement of Zeeman appeared, it seemed worth while to repeat the experiments with the