Ray Optics and Optical Instruments Part 3

A parallel beam of light travelling in air (refractive index 1.0) is incident on a convex spherical glass surface of radius of curvature 50 cm . Refractive index of glass is 1.5 . The rays converge to a point at a distance xcm from the centre of the curvature of the spherical surface. The value of x is ____ cm.