Q. 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 $cm$ away from the mirror
D) 0.8 $cm$ towards the mirror
Explanation
(a) 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 $0.1m$ then.
$u = (10 - 0.1) = 9.9cm$. 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 $cm$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.