On a horizontal plane mirror, a thin equiconvex lens of glass is placed and when the space between the lens and mirror is filled with a liquid, an object held at a distance D which is 30cm vertically above the lens is found to coincide with its own image as shown in Figure. If equiconvex lens of glass has refractive index `mu=1.5` and radius of curvature `R=20cm` then find refractive index of the liquid.