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 :

Answer 1

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

$v=-nu$

From mirror formula, $\frac{1}{u}+\frac{1}{v}=\frac{1}{f}$

$\frac{1}{u}+\frac{1}{-nu}=\frac{1}{f}$

$\frac{1}{u}(1-\frac{1}{n})=\frac{1}{f}$

$u=f\frac{(n-1)}{n}$