We have given \(\frac{\text { Size of the image }}{\text { Size of the object }}=n\)
In case of a concave mirror, if the image is real then it must be inverted.
So, \(\quad m=-n=\frac{-v}{u}\)
or \(\quad m=n=\frac{v}{u}\)
From mirror formula, we get
\(\frac{1}{u}+\frac{1}{v}=\frac{1}{f} \quad\) or \(\quad 1+\frac{u}{v}=\frac{u}{f}\)
or \(1+\frac{1}{n}=\frac{u}{f} \quad\) or \(\quad u=\left(\frac{n+1}{n}\right) f\)
