A parallel bean of light is incident normally on the flat surface of a hemisphere of radius 6 cm and refractive index 1.5, placed in air as shown in figure (i). Assume paraxial ray approximation.

The rays are focused at 12 cm from the point P to the right, in the situation as shown in figure (i)
 

The rays are focused at 16 cm from the point P to the right, in the situation as shown in figure (i) 
 

If the rays are incident at the curved surface (figure (ii)) then these are focussed at distance 18 cm from point P to the right. 
 

If the rays are incident at the curved surface (figure (ii)) then these are focussed at distance 14 cm from point P to the right.