CBSE Class 12-science Answered

In interfence the intensity at some points is greater than the sum of the intensity of the two sources and at other points it is also zero.

If you look at the equation of the wave formed by the superposition of waves from two sources you will see that to have a term in addition to the sum of the intensities of the two sources, the two sources must also have a constant phase difference phi. In that case the average of <cos ² Φ/2 >is cos ² Φ/2 . But if phi is changing then its average will be 1/2. This means that the total intensity is just the sum of the intensity of the two sources.