When two groups of waves (called wave trains) meet and overlap they
interfere with each other. The resulting amplitude will depend on the amplitudes of both the
waves at that point. The photograph shows some waves overlapping in a
fountain.
To make things simple let's just consider two waves overlapping and
assume that both the waves have the same amplitude.
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.
The diagrams in
Figure 1 below show two waves of equal amplitudes meeting. The first pair are in step and
the second pair are out of step.
You can see interference of water waves
quite easily in a ripple tank, on the sea or in a fountain like the one shown in the
photograph.
In the laboratory interference with sound can also be heard using two
speakers connected to the same signal generator and amplifier.
The following diagram shows the interference pattern produced by the overlapping sound waves.
