Question

A converging beam of rays is incident on a diverging lens. After passing through the lens, the rays intersect at a point $15cm$ from the lens. If the lens is removed, the point where the rays meet, moves $5cm$ closer to the mounting that holds the lens. Find the focal length of the lens.

• A. $-10cm$
• B. $-20cm$
• C. $-25cm$
• D. $-30cm$

Answer 1

The correct option is D $-30cm$

Using the lens formula
Here $u=10cm,v=15cm$
From lens formula,
$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}\frac{1}{f}=\frac{1}{15}-\frac{1}{10}=\frac{-1}{30}\Rightarrow f=-30cm$