one complete swing.^[1] The phase of any particle along the curve is the portion of a complete vibration which the particle has executed. The wave length is the distance between two particles in the same phase. Thus it is the distance FIG. 4between two consecutive crests or between two consecutive troughs. When all the particles vibrate in one plane, e. g., the plane of the drawing, the wave is said to be polarized in a plane. The velocity of propagation of the wave is the distance traveled by any given crest in one second.

As has just been stated, the type of wave motion illustrated in Fig. 4 may be approximately realized by imparting the motion of a pendulum or a tuning-fork to one end of a very long cord. It can be shown that after a time every particle of the cord will vibrate with precisely the FIG. 5same motion as that of the pendulum or tuning-fork from which the disturbance starts. Any particular phase of the motion occurs a little later in every succeeding particle; and it is this transmission of a given phase along the cord which constitutes the wave motion.