The superposition of two waves is illustrated in Fig. Figure 6a shows the waves just before superposition, Fig. 6b during superposition, and Fig. 6c after the waves have interacted. In constructive interference the amplitude of the composite pulse is the sum of the amplitudes of the individual pulses.
Waves can also interfere destructively.
Destructive interference is illustrated in Fig. 7. Sometimes they interfere constructively and at other times they interference destructively. Only when the two waves have the same wavelength or frequency, can they interfere constructively or destructively for all time, and, in addition, there must be a constant phase relation between the waves. Waves of the same wavelength and having a constant phase relationship are said to be coherent. Again, coherent waves must have the same frequency, and only coherent waves can interfere continuously, i.e., over long periods of time.