Question

A convex lens of focal length $10cm$ forms a virtual image at $30cm$ from the lens. Find the position of an object from the lens.

• A.
  $10cm$
• B.
  $8cm$
• C.
  $7.5cm$
• D.
  $75cm$

Answer 1

The correct option is C

$7.5cm$
Here $v=-30cmf=+10cmu=?$

So, from lens formula $\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$
We get,
$\frac{1}{-30}-\frac{1}{u}=\frac{1}{10}$
or $\frac{1}{u}=\frac{-1}{10}-\frac{1}{30}$
$\Rightarrow \frac{1}{u}=-\frac{4}{30}$
or $u=-7.5cm$

So, the object must be placed in front of the lens at a distance of $7.5cm$ from it.