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:
a. 5R
b. 3R
c. 2R
d. 1.5R