Ray Optics and Optical Instruments Part 2

A spherical surface of radius of curvature R, separates air from glass (refractive index =1.5 ). The centre of curvature is in the glass medium. A point object ' O ' placed in air on the optic axis of the surface, so that its real image is formed at ' γ ' inside glass. The line OI intersects the spherical surface at P and PO=PI. The distance PO equals to