Question

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

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

Answer 1

The correct option is B $\frac{(n-1)}{n}$f

Since Image is Real

Hence $M=+ve\Rightarrow M=n$
$M=\frac{-v}{u}=n$
$v=-un$
Using Mirror Formula
$\frac{1}{v}+\frac{1}{u}=\frac{1}{f}$
$\frac{1}{-\left(un\right)}+\frac{1}{u}=\frac{1}{f}$
$\frac{1+n}{un}=\frac{1}{f}$
$un=(n+1)f$
$u=\frac{(n+1)}{n}f$