# Interference

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.

You probably won't hear interference using your stereo or at a concert is because the sounds from the speakers are continually changing and what would be destructive interference at one moment at your ears would be constructive interference a moment later.

The reason that it is very difficult to see interference with light in everyday life is that the wavelength of light is very small.

### schoolphysics interference animation

To see an animation of interference click on the animation link.

### schoolphysics interference animation

To see an animation of interference in html5 click on the animation link.

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