Plane surface of plane - convex lens is silvered and it then acts like a concave mirror of focal length . 30 cm. If the refractive index of the lens is 1.5 , the radius of curvature of convex surface is

Question

$(a)\;15\;cm \\ (b)\;30\;cm \\ (c)\;60\;cm \\ (d)\;90\;cm $