A thin double convex lens is cut into two equal pieces A and B by a plane containing principal axis. The piece 'B' is further cut into two more pieces 'C' and 'D' by another plane perpendicular to the principal axis. If the focal power of the original lens is 'P', then those of A and C are

$$\begin{gathered} \mathrm{Cutting} \mathrm{a} \mathrm{lens} \mathrm{along} \mathrm{the} \mathrm{principal} \mathrm{axis} \mathrm{will} \mathrm{not} \mathrm{affect} \mathrm{its} \mathrm{focal} \mathrm{length}/\mathrm{power}. \\ \mathrm{So} \mathrm{the} \mathrm{power} \mathrm{of} \mathrm{pieces} \mathrm{A},\mathrm{B} \mathrm{will} \mathrm{be} \mathrm{P} \\ \mathrm{But} \mathrm{cutting} \mathrm{a} \mathrm{lens} \mathrm{perpendicular} \mathrm{to} \mathrm{the} \mathrm{principal} \mathrm{axis} \mathrm{will} \mathrm{effect} \mathrm{the} \mathrm{focal} \mathrm{length}. \\ \mathrm{The} \mathrm{focal} \mathrm{length} \mathrm{of} \mathrm{the} \mathrm{resulting} \mathrm{piece} \mathrm{will} \mathrm{be} \mathrm{half} \mathrm{of} \mathrm{the} \mathrm{initial} \mathrm{lens}. \\ \mathrm{So} \mathrm{the} \mathrm{power} \mathrm{of} \mathrm{the} \mathrm{pieces} \mathrm{B},\mathrm{C} \mathrm{will} \mathrm{be} \frac{\mathrm{P}}{2}. \end{gathered}$$