In YDSE arrangement as shown in figure, fringes are seen on screen using monochromatic source S having wavelength 3000 Å (in air). `S_1 `and `S_2` are two slits seperated by d = 1 mm and D = 1m. Left of slits `S_1` and `S_2` medium of refractive index `n_1 = 2` is present and to the right of `S_1` and `S_2` medium of `n_2 = 3/2`, is present. A thin slab of thickness 't' is placed in front of `S_1` . The refractive index of `n_3` of the slab varies with distance from it's starting face as shown in figure.

In order to get central maxima at the centre of screen, the thickness of slab required is :

Answer

2