On introducing a thin mica sheet of thickness $2\times {10}^{-6}\mathrm{ }\mathrm{m}$ and refractive index $1.5$ in the path of one of the waves, central bright maxima shifts by $n$ fringes. The wavelength of the wave used is $5000\mathrm{ }\stackrel{\circ }{\mathrm{A}}$ , then $n$ is