Page:Light waves and their uses.djvu/24

6 moving more slowly the smaller the wire and the heavier the blocks, so that, by varying these two factors, any desired speed may be obtained.

The wave form which is propagated in any of the various possible cases is, in general, very complicated. It can be shown, however, that it is always possible to express such forms, however complex, by a series of simple sine curves such as that represented in Fig. 4. The study of wave motion may be much simplified by this device. Accordingly, in all that follows, except where the contrary is expressly stated, it will be assumed that we are dealing with waves of this simple type.

There are certain characteristics of wave motion of which we shall have to speak frequently in what follows, and which therefore need definition. In the first place, the shape of the wave illustrated in Fig. 4 is important. It is the curve which would be drawn by a pendulum, carrying a marker, upon a piece of smoked glass moving uniformly at right angles to the motion of the pendulum. Since the pendulum moves in what is called simple harmonic motion, the curve is called a simple harmonic curve, or a sine curve. The amplitude of the wave is the maximum distance of a crest or a trough from the position of rest, i. e., from the straight line drawn through the middle of the curve. The period of the vibration is the time it takes one particle to execute one complete vibration; i. e., to revert to the pendulum, it is the time it takes the pendulum to execute