Question

The focal length of a convex lens is $20cm.$ If an object is kept at a distance of $100cm$ from the lens, find out the distance of the image formed from the lens.

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

Answer 1

The correct option is D $25cm$

Given:
Focal length, $f=20cm$
Note that focal length is positive for convex lens.
Object distance, $u=-100cm$

Let image distance be $v$.

Using lens formula:
$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$
$\frac{1}{v}-\left(\frac{1}{-100}\right)=\frac{1}{20}$
$v=25cm$

Therefore, image distance is $25cm$.