Probably everyone has, at some time or another, thrown a stone into a pond or other smooth sheet of water and noticed the circular ripples which spread out from the spot where the stone entered the water. These ripples are an example of a wave
motion travelling over a circular wavefront.
A somewhat simpler type of transverse wave, is seen when one end of a piece of rope or string is moved up and down in a direction perpendicular to its length. The particles of the rope near the end exert a drag on their neigh bours so that these begin to oscillate as well. This process continues throughout the rope, until finally any particular particle is oscillating up and down slightly later than the one immediately before it. The net result is that the rope presents the appearance of a series of equidistant crests and troughs which travel forward with a certain velocity, called the wave velocity (Fig. 26.l).
It is important to realize that it is only the shape or form of the wave which moves forward. The individual particles of the rope merely oscillate up and down with a motion similar to that of a pendulum bob. The actual motion of the particles has been indicated by small arrows in the diagram.
The maximum displacement of a particle from its rest position is called the
amplitude of the wave. The wavelength (J..) is defined as the distance between two successive particles which are at exactly the same point in their paths and are moving in the same direction. Such pairs of particles are said to be in the same phase. Examples are A and B or C and D (Fig. 26.1). Any line or section taken through an advancing wave in which all the particles are in the same phase is caUed the wavefront. We used this term earlier with reference to
circular water ripples.
The time taken for a wave particle to make one complete oscillation is called the periodic time. The number of complete oscillations made in 1 second is called the frequency (f). The SI unit of frequency is called the hertz (Hz) and is defined as 1 cycle (or oscillation) per second. In the time it takes the particles to make one complete oscillation the whole wave moves forward one wavelength. Hence in 1 second the wave moves forward a distance fA. But the distance moved per second is the velocity, v.
Hence v =fA