Focal length of a thin convex lens is 30 cm.  At a distance of 10 cm from the lens there is a plane refracting surface of refractive index 3/2.  Where will the parallel rays incident on lens converge?

The lens will converge the rays at its focus i.e., 30 cm from the lens or 10 cm from the refracting surface.

$\begin{array}{l}{\mathrm{PI}}_1=20\mathrm{cm}\\ \therefore {\mathrm{PI}}_2=\mathrm{\mu }\left({\mathrm{PI}}_1\right)=\frac{3}{2}\times 20=30\mathrm{cm}\\ \mathrm{So},\mathrm{the}\mathrm{rays}\mathrm{will}\mathrm{converge}\mathrm{at}\mathrm{a}\mathrm{distance}\mathrm{of}40\mathrm{cm}\mathrm{from}\mathrm{the}\mathrm{lens}.\end{array}$