State the importance of coherent sources in . the phenomenon of interference. In Young's double slit experiment to produce interference pattern, obtain the conditions for constructive and destructive interference. Hence deduce the expression for the fringe width ?

Coherent sources are those which produce coherent light which has a wavelength range restricted to very low range. So the rays of coherent light have almost equal wavelength. In lack of coherence interference pattern constantly change with time. In fact, the pattern change so rapidly that the human eye could not see the pattern. So to produce a stable and unchanging interference pattern, it is essential that the two sources be coherent.