In the set up shown in Fig the two slits, S₁ and S₂ are not equidistant from the slit S. The central fringe at O is then

Either dark or bright depending on the position of S

If path difference $\Delta=\left(S S_1+S_1 O\right)-\left(S S_2+S_2 O\right)=n \lambda n=0,1,2,3, ....$. the central fringe at O is a bright fringe and if the path difference $\Delta=\left(n-\frac{1}{2}\right) \lambda, n = 1, 2, 3, ....$. the central bright fringe will be a dark fringe.