Question

A converging beam of rays is incident on a diverging thin lens. Having passed through the lens the ray intersect at a point 15 cm from the lens. If lens is removed the point where the rays meet will move 5 cm closer towards the mounting that holds the lens. The focal length of lens is

• A. $f=10cm$
• B. $f=15cm$
• C. $f=30cm$
• D. $f=40cm$

Answer 1

Answer: (C)

$f=30cm$

$v=+15$ ; $u=+10$

$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$

$\frac{1}{15}-\frac{1}{10}=\frac{1}{f}$

$f=-30$
option $C$ is correct