A thin plano convex lens made of glass of refractive index \( 1.5 \) is immersed in a liquid of refractive index \( 1.2 \). When the plane side of the lens is silver coated for complete reflection, the lens immersed in the liquid behaves like a concave mirror of focal length \( 0.2 \, \text{m} \). The radius of curvature of the curved surface of the lens is:
