An equi-convex lens (R=10cm) has focal length equal to the focal length of a concave mirror (in magnitude). The refractive index of glass is . The radius of curvature of the concave mirror is

Focal length of convex lens 1f(n−1)1R1−1R2 R1=10cm,R2=−10cm,n=1.5 (for glass 1f(1.5−1)110−1(−10)=0.5210f=102×0.5=10cm∴Focal length of concave mirror = -10cm∴ Radius of curvature =2×(−10)=−20cm