Question

The given lens is broken into four parts and 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. $4 f $

Answer 1

Answer: (B)

Cutting a lens in transverse direction doubles their focal length i.e. $2 f$.
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 $f / 2$.