Question

A biconvex lens has a focal length of $10$ cm. It is cut in half two pieces are placed as shown. The focal length (in cm) of the final combination is

• A. $10$
• B. $20$
• C. $40$
• D. Not a lens

Answer 1

The correct option is A $10$

We know that on cutting the lens into two parts perpendicular to its principal axis power of the two parts will be $\frac{P}{2}$ each. Let, initial power of lens be P.
Then $(P_1{)}_f=(P_2)+f=\frac{P}{2}$
$P_f=(P_1{)}_f+(P_2{)}_f=P\therefore P_i=P_f$
No change in power hence no change in focal length.