Question

A concave mirror of focal length 'f' produces an image 'n' times the size of the object. If the image is real then the d distance of the object from the mirror is

• A. $(n+1)f$
• B. $\frac{(n+1)f}{n}$
• C. $\frac{(n-1)f}{n}$
• D. $(n-1)f$

Answer 1

Answer: (B)

$\frac{(n+1)f}{n}$

Since the image is real, it must be inverted and magnification ratio should be negative.

Mgnification $=\frac{-v}{u}$

$-n=\frac{-v}{d}$ , $u=d$ is the distance of object and and image distance is $v$

$v=nd$

Now using mirror formula

$\frac{1}{f}=\frac{1}{v}+\frac{1}{u}$

$\frac{1}{f}=\frac{1}{nd}+\frac{1}{d}$

So $d=\frac{f(n+1)}{n}$