In an experiment on Newton’s rings the light has a wavelength of 600 nm. The plano convex  lens has a refractive index of 1.5 and a radius of curvature of 2.5 m is placed on a  thick glass plate with curved surface in contact. If the radius of the 5th bright fringe formed for near normal incidence and in the reflected light approximately is  $\left(1.3\right)n\times {10}^{-3}m$  then n is