Question
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 $..........\ cm .$
✓
Answer
$v=3 u$
$v - u =20 \ cm$
$2 u =20 \ cm$
$u =10 \ cm$
$\frac{1}{(-30)}-\frac{1}{(-10)}=\frac{1}{ f }$
$f =15 \ cm$
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