Question

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

• A. $36cm$
• B. $25cm$
• C. $30cm$
• D. $150cm$

Answer 1

Answer: (A)

$36cm$

Let the object distance be $x$, i.e $u=-x$

Magnification, $m=\frac{-v}{u}$

$\Rightarrow$ $5=\frac{-v}{-x}$

$\Rightarrow v=5x$

Using lens formula:

$\Rightarrow \frac{1}{v}-\frac{1}{u}=\frac{1}{f}$

$\Rightarrow \frac{1}{5x}-\frac{1}{-x}=\frac{1}{30}$

$\Rightarrow$$\frac{6}{5x}=\frac{1}{30}$

$\Rightarrow x=36$ $cm$

Thus the object distance is $36$ $cm$.