In a YDSE experiment if a slab whose refractive index μ can be varied is  placed in front of one of the slits, then the variation of resultant intensity at midpoint of screen with 'μ' will be best represented by μ≥1 . [Assume slits of equal width and there is not absorption by slab].

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.