Question

A convex lens of focal length ${}^{\prime }f{}^{\prime }$ cut into parts first horizontally and then vertically. Find the focal length of part A of the lens, as shown

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

Answer 1

The correct option is D $2f$

initially $f=R$

for part A,

$\frac{1}{f^{{}^{\prime }}}=\frac{3/2-1}{R}=\frac{1}{2R}$

$f^{{}^{\prime }}=2R=2f$

option $D$ is correct