Question

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

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

Answer 1

The correct option is A 0.4cm 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 0.1cm then.

u=10-0.1=9.9 cm. Hence again from mirror formula $\frac{1}{-\frac{20}{3}}=\frac{1}{v^{\prime }}+\frac{1}{-9.9}\Rightarrow$ v'=20.4cm

i.e. images shift away from the mirror by 0.4 cm.