When there is a single source of light, the distribution of light energy in all directions will be uniform. But when we have two coherent sources of light, emitting light waves of same amplitude, same frequency and in the same phase, then the distribution of light energy does not remain uniform in all the directions. At some points, where crest of 1 wave falls on the crest of other wave, the resultant intensity of light will be maximum and at the other points, where crest of 1 wave falls on the trough of other wave, then the intensity of light is minimum. This kind of modification in the energy distribution of light is called Interference of light. At points where resultant intensity of light is maximum is said to be constructive interference and at points, where the resultant intensity of light is minimum, is said to be destructive interference.