Question

If the separation between the slits in Young's double slit experiment is increased, what happens to the fringe width? If the separation is increased too much, will the fringe pattern remain detectable?

Answer 1

In Young's double slit experiment the fringe width is given as w = Dλ/d, where d is the separation between the slits. Clearly, if d is increased the fringe width decreases.      

If the separation d is increased too much the fringe width will be very small. Now the maxima and minima will be so closely spaced that it will look uniform intensity pattern.