Question

An object is placed at a distance of $30cm$ from the lens. Its image is formed at a distance of $10cm$ on the same side of the lens. Find the focal length of the lens used.

• A. $f=10cm$
• B. $f=-10cm$
• C. $f=+15cm$
• D. $f=-15cm$

Answer 1

The correct option is D $f=-15cm$

Given:
Image distance $v=-10cm$
Object distance $u=-30cm$
(Using the sign conventions)

Let focal length of the lens be $f$.

Using the lens formula,
$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$
$\frac{1}{f}=\left(\frac{1}{-10}\right)-\left(\frac{1}{-30}\right)$
$f=-15cm$

Here, negative sign for focal length shows that the lens is concave.