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  • At what distance from a concave mirror of focal length 10 cm should an object 2 cm long be placed to get an erect image 6 cm tall?

At what distance from a concave mirror of focal length 10 cm should an object 2 cm long be placed to get an erect image 6 cm tall?

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Given that focal length, f= -10cm

And  m=\frac{Image \ height}{Object \ height}=\frac{6}{2}=3

also m= \frac{-v}{u}

So we get v= -3u

Using 

\frac{1}{v}+\frac{1}{u}=\frac{1}{f}

\frac{1}{-3u}+\frac{1}{u}=\frac{1}{f}\Rightarrow \frac{2}{3u}= \frac{1}{f}

as f=-10

Therefore u= -6.667

So the object should be placed at a distance of 6.667 cm in front of the mirror.

Posted by

avinash.dongre

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