Question

A convex lens of focal length $5cm$ is placed at a distance of $6cm$ from a wall. How far from the lens should an object be placed so as to form its real image on the wall?

• A. $30cm$
• B. $-30cm$
• C. $1.2cm$
• D. $-1.2cm$

Answer 1

The correct option is B $-30cm$

Given:

Focal length of lens,$f=5cm$

Distance between lens and screen = distance of image formed from the lens$=v=6cm$

We know, $\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$

$\frac{1}{6}-\frac{1}{u}=\frac{1}{5}$
$\frac{1}{6}-\frac{1}{5}=\frac{1}{u}$

$\frac{5-6}{30}=\frac{1}{u}$
$\frac{1}{u}=\frac{-1}{30}$

Therefore, the distance of the object from the lens, $u=-30cm$. Or, the object should be placed $30cm$ far from the lens on the opposite side of the wall.