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Q.
A convex lens of focal length $0.12\, m$ produces an image, which is three times as long as the object. The distance between the object and the lens for a real image
ManipalManipal 2020
Solution:
Magnification produced by the lens
$m=\frac{v}{u}=-3 \Rightarrow v=-3\, u$
Applying lens of formula, we have
$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$
$\Rightarrow \frac{1}{-3 u}-\frac{1}{u}=\frac{1}{0.12}$
or $\frac{-1-3}{3 u}=\frac{100}{12}=\frac{25}{3}$
or $-\frac{4}{3 u}=\frac{25}{3}$
or $u=-\frac{4}{25} m=0.16\, m$