A point object is placed at a distance of 10cm and its real image is formed at a distance of 20 cm from a concave mirror. If the object is moved by 0.1cm towards the mirror, the image will shift by about

Mirror formula 

$\frac{1}{\mathrm{f}}=\frac{1}{\mathrm{v}}+\frac{1}{\mathrm{u}}\Rightarrow \frac{1}{\mathrm{f}}=\frac{1}{-20}+\frac{1}{(-10)}\Rightarrow \mathrm{f}=\frac{20}{3}\mathrm{cm}$

If object moves towards the mirror by   then. 

$\mathrm{u}=(10-0.1)=9.9\mathrm{cm}$. 

Hence again from mirror formula $\frac{1}{-20/3}=\frac{1}{{\mathrm{v}}^{\mathrm{\prime }}}+\frac{1}{-9.9}\Rightarrow {\mathrm{v}}^{\mathrm{\prime }}=20.4\mathrm{cm}$ 

i.e., image shifts away from the mirror by 0.4 cm.