An object is placed at a distance of 40 cm from a concave mirror of focal length 15 cm. If the object is displaced through a distance of 20 cm towards the mirror, the displacement of the image will be
$\dfrac{1}{f}= \dfrac{1}{v_{1}}+\dfrac{1}{u}$
$-\frac{1}{15}=\dfrac{1}{v_{1}}-\dfrac{1}{40}$
$\Rightarrow\dfrac{1}{v_{1}}= -\frac{1}{15}+\dfrac{1}{40}$
$v_{1}=-24cm$
When object is displaced by 20 cm towards mirror.
Now, $u_{2}=-20$
$\dfrac{1}{f}=\dfrac{1}{v_{2}}+\dfrac{1}{u_{2}}$
$\dfrac{1}{-15}=\dfrac{1}{v_{2}}-\dfrac{1}{20}$
$\dfrac{1}{v_{2}}= \dfrac{1}{20}-\dfrac{1}{15}$
$v_{2}=-60cm$
So, image shifts away from mirror by = 60 – 24 = 36 cm.