Question

A convex lens of focal length $30cm$ produces $5$ times magnified real image of an object. What is the object distance?

• A. $36cm$
  
• B. $25cm$
  
• C. $30cm$
  
• D. $125cm$
  

Answer 1

The correct option is A $36cm$

Focal length of the convex lens $f=30cm$

A convex lens forms a magnified real image when the image is inverted i.e. magnification must be negative.

Magnification of the lens $m=-5$

Using lens formula $\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$

We get $\frac{v}{f}=1-m$

$\therefore$ $\frac{v}{30}=1-(-5)$

$⟹$ $v=180cm$

Using $m=\frac{v}{u}$ (for lens)

$\therefore$ $-5=\frac{180}{u}$

$⟹$ $u=-36cm$

Thus the object distance must be $36cm$ in front of the lens.