A plano-convex lens when silvered in the plane side behaves like a concave mirror of focal length 30 cm. However, when silvered on the convex side it behaves like a concave mirror of focal length 10 cm. Then the refractive index of its material will be
(d) Here
Plano-convex lens silvered on plane side has .
Plano-convex lens silvered on convex side has
Now using , we get