Thus knowing m(order of minima), , D and y (separation of m'th minima from principal maxima ) the slit width 'd' can be determined. Now, let us see, when we will observe maxima in diffraction pattern. At the center of screen, the path traveled by the secondary waves originating from one point of slit is same as that traveled by another wave on the opposite side of center of slit at the same distance and hence path difference is zero