Question:
When an object is kept at a distance of $30 \mathrm{~cm}$ from a concave mirror, the image is formed at a distance of $10 \mathrm{~cm}$ from the mirror. If the object is moved with a speed of $9 \mathrm{cms}^{-1}$, the speed (in $\mathrm{cms}^{-1}$ ) with which image moves at that instant is______
Solution:
(1)
Distance of object, $u=-30 \mathrm{~cm}$
Distance of image, $v=10 \mathrm{~cm}$
Magnification, $m=\frac{-v}{u}=\frac{(-10)}{-30}=\frac{1}{3}$
Speed of image $=m^{2} \times$ speed of object $=\frac{1}{9} \times 9=1 \mathrm{~cm} \mathrm{~s}^{-1}$