Wave Motion and Interference
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I believe that I shall be much more likely to interest you by telling what I know, than by repeating what someone else knows.
In order to discuss intelligently these applications of light waves, it will be necessary to recall some fundamental facts about light and especially about wave motion. These facts, though doubtless familiar to most of us here, need emphasis and illustration in order that we may avoid, as far as possible, the tedious repetition against which we were warned.
Doubtless there are but few who have not watched with interest the circular waves produced by a stone cast into a still pond of water, the ever-widening circles, going farther and farther from the center of disturbance, until they are lost in the distance or break on the shore. Even if we had no knowledge of the original disturbance, its character, in a general way, might be correctly inferred from the waves. For instance, the direction and distance of the source can be determined with considerable accuracy by drawing two lines perpendicular to the front of the wave; the source would lie at their intersection. The size of the waves will give information concerning the size of the object thrown. If the waves continue to beat regularly on the shore, the disturbance is continuous and regular; and, if regular, the frequency (/. <?», the number of waves per second) determines whether the disturbance is due to the splash of oars, to the paddles of a steamer, or to the wings of an insect struggling to escape.
In a precisely similar manner, though usually without conscious reasoning about the matter on our part, the sound waves which reach the ear give information regarding the source of the sound. Such information may be classified as follows:
1. Direction (not precise).
2. Magnitude (loudness).
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Light Waves and Their Uses
3. Frequency (pitch).
4. Form (character).
Light gives precisely the same kinds of information, and hence it is only natural to infer that light also is a wave motion. We know, in fact, that it is so; but before giving the evidence to prove it, it will be well to make a little preliminary study of the chief characteristics of wave motion.*
FIG. 1
One of the difficulties encountered in studying wave motion is the rapidity of the propagation of the waves. A fairly moderate speed is attained by the waves propagated along a spiral spring. If one end of such a spring be fastened to a wooden box on the wall of the lecture-room, while the other end is held in the hand, we can see that any motion communicated by the hand is successively transmitted to the different parts of the spring until it reaches the wall. Here it is reflected back toward the hand, but with diminished amplitude. We can also see that any kind of transverse motion, /. f\, motion at right angles to the length of the spring, whether regular or irregular, gives rise to a corresponding wave form which travels along the spring with a velocity that is the same in every case.
If the spring be very suddenly stretched or relaxed, a