Question

A Convex lens of focal length $10cm$ is placed at a diatance of $20cm$ from a wall. How far from the lens should an object be placed so as to form its real image on the wall ?

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

Answer 1

The correct option is B

$20cm$
$f=10cmv=20cm$

Using lens formula
$1/v-1/u=1/f$
$1/u=1/v-1/f$
$1/u=1/20-1/10$

$u=-20cm$

or
Image is formed at $2f$ hence the object must be placed at $2f$ distance from the lens.