It is found that it is not possible to show interference due to two independent sources of light because a large number of difficulties are involved. Two sources may emit light wave of different amplitude and wavelength and the phase difference between the two may change with time.
The fundamental requirement to get will defined interference pattern is that the phase difference between the two waves should be constant. The two sources are said to be coherent if they emit light waves of same frequency nearly the same amplitude and are always in phase with each other. In actual practice it is not possible to have two independent sources which are coherent.
But for experimental purpose two virtual sources formed from a single source can act as coherent sources. Methods for producing coherent sources is divided in following two parts have been divide (i) interference of light take place between waves from two sources formed due to single source (division of wavefront) (ii) interference takes place between the waves from the real source and virtual source (division of amplitude).