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 from the mirror. If the object is moved with a speed of 9\; cms^{-1}, the speed (in\; cms^{-1}) with which image moves at that instant is _____.
Option: 1 1
Option: 2 2
Option: 3 3
Option: 4 4

Answers (1)

For concave mirror
 

\frac{1}{v}+\frac{1}{u}=\frac{1}{f} \\ And \ |\mathrm{m}|=\left|-\frac{\mathrm{v}}{\mathrm{u}}\right|=\frac{10}{30}=\frac{1}{3} \\ Now using \\ v_{\text {image }}=v_{\text {object }}\left(m^{2}\right) \\\Rightarrow v_{\text {image }}=9 \times \frac{1}{9}=1 \mathrm{~cm} / \mathrm{s}

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