Interference:
We have discussed how particles of the medium get displaced from their equilibrium position when a wave passes across them. Now (as we are going to look at properties of waves interference), we will see that when particles of a medium are subjected to two waves simultaneously an interesting phenomenon takes place known as “interference”.

If two identical waves propagate through a medium in such a way that crest of one wave falls on the crest of the other and trough of the one falls on the trough of the other, the resultant displacements of particles when the two waves simultaneously act upon them get double the displacement of the individual wave. Such an interference has been shown in fig. 12.16-a. in this figure one wave has been shown by dotted line while the other with solid line and they are shown to be helping each other and generating a single wave of double the amplitude. Such interference is called constructive interference.

On the other hand, if two identical waves act upon the particles of the medium in such a way that the crest of the one falls on the trough of the other, they cancel the effect of each other and as a result the surface of the medium remains undisturbed. Such an interference is called destructive interference (Fig 12.16-b).