Q. When an object is kept at a distance of 30 cm from a concave mirror, the image is formed at a distance of 10 cm. If the object is moved with a speed of 9 $ms^{- 1}$ , the speed with which images moved is
A) 0.1$\ ms^{- 1}$
B) 1$\ ms^{- 1}$
C) 3$\ ms^{- 1}$
D) 9$\ ms^{- 1}$
Explanation
(b) $\frac{1}{u} + \frac{1}{v} = \frac{1}{f}$
$- \frac{du}{u^{2}} - \frac{dv}{v^{2}} = 0$
or$\ - \frac{dv}{v^{2}} = \frac{du}{u^{2}}$
Or $\ \frac{dv}{dt} = - \frac{v^{2}}{u^{2}}\frac{du}{dt} = - \frac{10 \times 10}{30 \times 30} \times 9\ ms^{- 1} = - 1\ ms^{- 1}$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.