Question

A convex lens of focal length F forms a real image, 3 times the size of object. The distance of object from the lens is

• A. F
• B. $\frac{3}{2}F$
• C. $\frac{4}{3}F$
• D. $\frac{F}{3}$

Answer 1

The correct option is C $\frac{4}{3}F$

Given: Focal length = F
Magnification, $m=\frac{v}{u}=\frac{h\text{'}}{h}=-3$$\Rightarrow v=-3u$
Using the lens formula, $\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$
$\Rightarrow \frac{1}{-3u}-\frac{1}{u}=\frac{1}{F}$
$\Rightarrow u=-\frac{4}{3}F$
So, distance of the object from the lens is $\frac{4}{3}F$.
Hence, the correct answer is option (3).