An object is kept at a distance of $20cm$ from a converging lens of focal length $10cm$. What will be the magnification?

Given: u=-20 cm        f=10 cm     (concave lens)
To find : magnification
Solution : From Lens Formula
$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$
$\frac{1}{v}+\frac{1}{20}=\frac{1}{10}$
$\frac{1}{v}=\frac{1}{10}-\frac{1}{20}$
$\frac{1}{v}=\frac{1}{20}$
$v=20cm$
So magnification for a lens is given by
$m=\frac{v}{u}=\frac{20}{-20}=1$
$m=-1$