When μ=1 i.e., no change in path difference due to optical path difference so the mid point of the screen is again the central bright fringe. But when μ>1 ,  the central bright fringe will shift according to Ddμ−1t. As μ increases, central bright fringe will shift upwards from midpoint i.e., at mid point, less bright fringe appears and when the shift is equal to value of fringe width, the dark fringe (zero intensity) will appear.