Wave optics

Ans:4
Please explain how to solve this sum. Please show working.

Answer is d/6. Just a simple geometry. As distance between the slits is d so At d/2 central bright fringe will form. And now we can say if x is the distance of that central bright fringe from O then 2d/3 = d/2 + x so x= d/6 . And even in options this is the minimum answer given and 0 we can't be sure of as other conditions aren't given to find out the fringe width so that we can check if 0 is possible or not.