Figure 18.1 is a pictorial representation of the superposition of two pulses. The
wave function for the pulse moving to the right is y1, and the wave function for the
pulse moving to the left is y2. The pulses have the same speed but different shapes, and
the displacement of the elements of the medium is in the positive y direction for both
pulses. When the waves begin to overlap (Fig. 18.1b), the wave function for the resulting complex wave is given by y1 y2. When the crests of the pulses coincide
(Fig. 18.1c), the resulting wave given by y1 y2 has a larger amplitude than that of the
individual pulses. The two pulses finally separate and continue moving in their original
directions (Fig. 18.1d). Note that the pulse shapes remain unchanged after the interaction, as if the two pulses had never met!