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  • A converging mirror forms a real image of height 4cm of an object of height 1cm placed 20cm away from the mirror. Calculate the focal length of the mirror.

A converging mirror forms a real image of height 4cm of an object of height 1cm placed 20cm away from the mirror. Calculate the focal length of the mirror.

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\begin{array}{l} h_{2}=-4 \mathrm{cm} \text { (real image which means inverted image) } \\ h_{1}=1 \mathrm{cm} \\ \mathrm{u}=-20 \mathrm{cm} \end{array}

Magnification \ m=\frac{-v}{u}$ $-4=\frac{-v}{-20}$ $v=-80 \mathrm{cm}$

From mirror formula

\begin{array}{l} \frac{1}{v}+\frac{1}{u}=\frac{1}{f} \\ \frac{1}{-80}+\frac{1}{-20}=\frac{1}{f} \\ \frac{1}{f}=-\frac{1}{80}-\frac{1}{20}=\frac{-1-4}{80}=-\frac{5}{80} \\ f=-16 \mathrm{cm} \end{array}

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avinash.dongre

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