Question

An object is placed at $30cm$ in front of the convex lens of focal length $10cm$. Find out the image distance from lens?

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

Answer 1

The correct option is B

$+15cm$

Given:
Object distance, $u=-30cm$
Focal length, $f=10cm$ (According to sign conventions)
Let image distance be $=v$
From lens formula,
$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$
$\frac{1}{v}-(-\frac{1}{30})=\frac{1}{10}$
$\frac{1}{v}+\frac{1}{30}=\frac{1}{10}$
$\frac{1}{v}=\frac{1}{10}-\frac{1}{30}=+\frac{2}{30}$
$v=+15cm$
Real, inverted and diminished image is formed.