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

Question

TestHub · Accepted Answer

The image will be formed on the same side as the object. Therefore, $v = 3 u$

Given: $|v - u| = 20 cm$

$\Rightarrow |2 u| = 20 cm$

$\Rightarrow u = - 10 cm$ (-ve sign as the object is real)

From lens formula, we can write

$\frac{1}{(- 30)} - \frac{1}{(- 10)} = \frac{1}{f}$

$\Rightarrow f = 15 cm$

Physics - Geometrical Optics Question with Solution | TestHub

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