Question

When an object is kept at a distance of $30\text{cm}$ from a concave mirror, the image is formed at a distance of $10\text{cm}$ from the mirror. If the object is moved with a speed of $9cm{s}^{-1}$, the speed (in $cm{s}^{-1}$) with which the image moves at that instant is .

Answer 1

$\text{Distance of object,}u=-30\text{cm}$
$\text{Distance of image,}v=10\text{cm}$

$\text{Magnification,}m=\frac{-v}{u}=\frac{(-10)}{-30}=\frac{1}{3}$

$\text{Speed of image}=m^2\times \text{speed of object}$

$\Rightarrow \text{Speed of image}=\frac{1}{9}\times 9=1{\text{cms}}^{-1}$