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  4. 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 .

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Q. When an object is kept at a distance of $30 \,cm$ from a concave mirror, the image is formed at a distance of $10 \,cm$ 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 _______.

JEE MainJEE Main 2020Ray Optics and Optical Instruments

Solution:

$\left|\left(\frac{dv}{dt}\right)\right|=\left|\frac{v^{2}}{4^{2}}\right|\left|\frac{du}{dt}\right|=\left(\frac{10}{30}\right)2\times9=1 m /s$