The radius of curvature of a thin plano-convex lens is 10cm (of curved surface) and the refractive index is 1.5. If the plane surface is silvered, then it behaves like a concave mirror of focal length (in cm )

The silvered plano convex lens behaves as a concave  mirror; whose focal length is given by 1 F=2f1+1fm If plane surface is silvered ,fm=R22=∞2=∞1f1=(μ−1)(1R1−1R2)=(μ−1)(1R−1∞)=μ−1R ∴1 F=2(μ−1)R+1∞=2(μ−1)R⇒F=R2(μ−1) Here R=20 cm,μ=1.5∴F=202(1.5−1)=20 cm