Question

A concave mirror of focal length $f$ produces a real image $n$ times the size of the object. The distance of the object from the mirror is

• A. $(n-1)f$
• B. $(n+1)f$
• C. $(n+1)f/n$
• D. $(n-1)f/n$

Answer 1

Answer: (C)

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
$<br/>\begin{array}{l}<br/>\frac{1}{f}=\frac{1}{v}+\frac{1}{u}\\ <br/>\frac{1}{f}=\frac{1}{nd}+\frac{1}{d}\\ <br/>\text{ So }d=\frac{f(n+1)}{n}<br/>\end{array}<br/>$