With a single source of light the distribution of light energy in the surrounding pace is uniform but when there are two such sources, under certain condition the distribution is no longer uniform. There are certain regions where on account of superposition of waves enhanced light intensity is observed while at certain other places the two set of waves destroy each other so darkened is produced. This modification of the intensity of light distribution obtained by the superposition of two or more waves under certain condition is called phenomenon of Interference.
The two set of waves coming from the two sources cross each other than they pass through any point. The presence of one wave does not affect the flow of other waves but total amplitude at any point will be the algebraic sum of the amplitudes of parent waves. At certain point the wave superimpose in such a way that resultant intensity is greater than the sum of intensities due to individual waves. The interference is called constructive interference. While at certain other points the resultant intensity is less than the sum of the intensities due to individual waves-the phenomenon is called destructive interference. It must be clearly understood that there is no destruction of energy anywhere. Energy is merely redistributed so that energy missing at one point appears at another point. The interference phenomenon was demonstrated experimentally by Thomas Young for the first time which established that light travel in the form of wave.