The focal length of a thin biconvex lens is 20cm. when an object is moved from a distance of 30cm in front of it to 50cm, the magnification of its image changes from $m_{30}$ to $m_{50}$ . The ratio $\frac{m_{30}}{m_{50}}$ ?

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answer is 3.

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Detailed Solution

Magnification of lens is given as

$\begin{array}{l} m = \frac{f}{f + u} \\ \Rightarrow m_{25} = \frac{20}{20 - 30} = - 2; \\ a n d m_{50} = \frac{20}{20 - 50} = - \frac{2}{3} \\ \Rightarrow \frac{m_{25}}{m_{50}} = 3 \end{array}$

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