When two groups of waves meet and overlap they interfere with each other. The resulting amplitude will depend on the amplitudes of both the waves at that point.

If the crest of one wave meets the crest of the other the waves are said to be in step (or in phase). The two sets of waves ‘add together’ and the resulting intensity will be large. This is known as constructive interference.

If the crest of one wave meets the trough of the other they are said to be out of step (or out of phase). The two sets of waves cancel each other out and the resulting intensity will be zero. This is known as destructive interference.