Question

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

• A. 0.4 cm away from the mirror
• B. 0.4 cm towards the mirror
• C. 0.8 away from the mirror
• D. 0.8 cm towards the mirror

Answer 1

The correct option is A 0.4 cm away from the mirror

Mirror formula
$\frac{1}{f}=\frac{1}{v}+\frac{1}{u}\Rightarrow \frac{1}{f}=\frac{1}{-20}+\frac{1}{(-10)}\Rightarrow f=\frac{20}{3}cm.$
If object moves towards the mirror by 0.1 cm then. u =(10-0.1)=9.9 cm.
Hence again from mirror formula $\frac{1}{\frac{-20}{3}}=\frac{1}{v}+\frac{1}{-9.9}\Rightarrow v^{\prime }=20.4cm$
i.e. image shifts away from the mirror by 0.4 cm.