The plane surface of a plano convex lens having radius of curvature 20 cm is silvered. If this silvered lens is equivalent to a concave mirror, the radius of curvature of the mirror is (Refractive index of glass is 1.5)

Focal length of a silvered lens is given by 1F=−2fl+1fmThe silvered surface behaves like a plane mirror, therefore fm=∞. Also 1fl=μ−1R=1.5−120=140∴ 1F=−2×140⇒F=−20 cmSo focal length of the concave mirror is 20 cm. Hence radius of curvature =2×20 cm=40 cm