Question

A biconvex lens of focal length $f$ is cut into two plano convex lenses. If these two plano convex lenses are joined such that their convex surfaces are in contact, then the focal length of the combination is :

• A. $\frac{f}{2}$
• B. $f/4$
• C. $2f$
• D. $\text{infinite}$

Answer 1

The correct option is B $f/4$

$f_1=f/2$ $f_2=f/2$
$\frac{1}{f^{{}^{\prime }}}=\frac{1}{f_1}+\frac{1}{f_2}$

$\frac{1}{f^{{}^{\prime }}}=\frac{2}{f}+\frac{2}{f}$

$f^{\prime }=f/4$