Question

If s is the size of the source and its distance from the plane of the two slits, what should be the criteria for the interference fringes to be seen?

Answer 1

If s is the size of the source and d is its distance from the plane of the two slits, the criteria for the interference fringes to be seen is

sd < λ/d

other necessary conditions are :

1) d should be small, i.e. two slits should be close to each other

2) The source should be a monochromatic source producing two coherent light waves

I hope this helps you, feel free to ask any doubts in the comment section.