that of the standing wave corresponding to the fourth harmonic shown in Figure 6.2.
As time increases the resultant standing wave evolves as shown in Figure 6.3. Any
point on the standing wave is described by Equation (6.22), i.e. y = A sin kx cos ωt.
The transverse displacement of every point on the standing wave varies with SHM
as cos ωt and the amplitude of this motion varies as A sin kx, i.e. the nodes and
antinodes occur at fixed points on the x-axis, cf. discussion of Equation (6.1).
The two travelling sinusoidal waves that we have considered above extend to
large distances in both directions (in principle to x = ±∞). A string stretched
between two rigid walls has a finite length. However, it can still support standing
waves. In this case it is reflections at the two walls that produce the two waves
travelling in opposite directions. This is illustrated in Figure 6.4, which shows the
formation of a standing wave on a string stretched between two rigid walls. The
figure represents snapshots of the waves, at successive instants of time, separated
by T /8, where T is the period of the waves. Again the thin continuous curve
represents a wave travelling towards the right and the dotted curve represents a
wave travelling towards the left. (At some instants of time, the incident and reflected
waves lie on top of each other.) These waves are reflected at each of the walls.
Inspection of Figure 6.4 shows that the waves obey the rules of reflection that we