A spherical surface of radius of curvature ‘R’ separates air (refractive index 1.0) from glass (refractive index 1.5). The centre of curvature is in the glass. A point object ‘P’ placed in air is found to have a real image ‘Q’ in the glass. The line PQ cuts the surface at a point ‘O’, and PO = OQ. The distance PO is equal to