Question 160 of 295

The distance between object and its $3$ times magnified virtual image as produced by a convex lens is $20cm.$ The focal length of the lens used is ________ $cm.$

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Answer: 15

The image will be formed on the same side as the object. Therefore, $v=3u$
Given: $\left|v–u\right|=20cm$
$\Rightarrow \left|2u\right|=20cm$
$\Rightarrow u=-10cm$ (-ve sign as the object is real)
From lens formula, we can write
$\dfrac{1}{(-30)}-\dfrac{1}{(-10)}=\dfrac{1}{f}$
$\Rightarrow f=15cm$

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