Question

The given lens is broken into four parts rearranged as shown. If the initial focal length is $f$, then after rearrangement the equivalent focal length is

• A. $f$
• B. $f/2$
• C. $f/4$
• D. $4f$

Answer 1

Answer: (B)

$f/2$

Cutting a lens in transverse direction doubles their focal length, i.e., $2f$.
Using the formula of equivalent focal length,
$\frac{1}{f}=\frac{1}{f_1}+\frac{1}{f_2}+\frac{1}{f_3}+\frac{1}{f_4}$
We get equivalent focal length as $\frac{f}{2}$.