In double slit experiment, a light of wavelength $\lambda =600nm$  is used. When a film of material $3.6\times {10}^{-3}cm$   thick was placed over one of the slits, the fringe pattern was displaced by a distance equal to  60  times that between two adjacent fringes. What is the refractive index of the martial ?

Wavelength $\lambda =600nm=600\times {10}^{-9}m$

Thickness $t=3.6\times {10}^{-3}cm=3.6\times {10}^{-5}m$

Number of fringes $n=60$

Refractive index $\mu =?$

                        $\mu =\frac{n\lambda }{t}+1$

                         $=\frac{60\times 600\times {10}^{-9}}{36\times {10}^{-6}}+1$

                        $=\frac{36000\times {10}^{-9}}{36\times {10}^{-6}}+1$

                        $=\frac{36\times {10}^{-6}}{36\times {10}^{-6}}+1$

                  $\therefore \mu =2$