In youngs double slit experiment if S is the size of source & d is its distance from the plane of the 2 slits , what should be the criterion for interference fringes to be seen?

A distance d from a thermal monochromatic (line) source whose linear dimensions are on the order of S, two slits separated by a distance greater than 0.16λd/S will no longer produce a recognizable interference pattern. This is called spatial coherence condition.