One end of a glass rod of refractive index

n=1.5 is a spherical surface of radius of curvature

R. The centre of the spherical surface lies inside the glass. A point object placed in air on the axis of the rod at the point

P has its real image inside glass at the point

Q (see fig.). A line joining the points

P and

Q cuts the surface at

O such that

OP=2OQ. The distance

PO is.