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

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 $\frac{1}{- 20 / 3} = \frac{1}{v'} = \frac{1}{- 9.9} \Rightarrow v' = 20.4 cm$ .

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

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