Light of wavelength $\lambda$ is incident normally on a thin film. The reflected light shows a minimum for reflection at the minimum possible thickness. The same film is placed in front of the upper slit of a YDSE apparatus. The fringe pattern shifts up by $7.5\,\text{mm}$. The refractive index of the material is (Take: $D = 5\,\text{m}$, $d = 0.1\,\text{mm}$, $\lambda = 500\,\text{nm}$):