Question

An object is placed $15cm$ in front of a lens. The lens forms a real image three times magnified. Find out the image distance and the focal length of a lens?

• A. $v=35cm$
  $f=13.25cm$
• B. $v=45cm$
  $f=11.25cm$
• C. $v=-35cm$
  $f=-13.25cm$
• D. $v=-45cm$
  $f=11.25cm$

Answer 1

The correct option is B $v=45cm$

$f=11.25cm$
Given:
Object distance, $u=-15cm$
Magnitude of magnification, $|m|=3$
Image produced is real.

Since the image produce by the lens is real, it is a convex lens, image distance is positive and magnification is negative.

Magnification is given by:
$m=\frac{v}{u}$
$-3=\frac{v}{-15}$
$v=45cm$

Using Lens formula,
$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$
$\frac{1}{f}=\frac{1}{45}-\frac{1}{(-15)}$
$f=11.25cm$