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

If path difference Δ=SS1+S1O−SS2+S2O=nλ; n = 0, 1, 2, 3, …. the central fringe at O is a bright fringe and if the path difference Δ=n−12λ  n = 1,2,3, ..... the central bright fringe will be a dark fringe.