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

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Solution

Q.
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. [2024]

Ans.
(15)

$m = \frac{v}{u} \Rightarrow v = 3 u$

$∵$ $v - u = 20$

$3 u - u = 20 \Rightarrow u = 10 c m,$ $v = 30 c m$

$\frac{1}{f} = \frac{1}{v} - \frac{1}{u} = - \frac{1}{30} + \frac{1}{10} = \frac{- 1 + 3}{30} = \frac{2}{30} \Rightarrow f = 15 cm$

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