Question

If an object is placed at a distance of $30cm$ and the image is formed at a distance of $10cm$ on the same side of the lens, then find the focal length of the lens used.

• A. $10cm$
• B. $15cm$
• C. $20cm$
• D. $25cm$

Answer 1

The correct option is B $15cm$

Given:
Image distance, $v=-10cm$
Object distance, $u=-30cm$
[Using the sign conventions, as both object and image are on the left side of the lens] Let focal length be $f$.

Using the lens formula:
$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$
$\frac{1}{-10}-\frac{1}{-30}=\frac{1}{f}$
$f=-15cm$
Here negative sign shows that the lens is concave.