Question

A convex lens of focal length $0.12m$ produces an image, which is three times as long as the object. The distance between the object and the lens for a real image

• A. 0.16 m
• B. -0.16 m
• C. 1.6 m
• D. -1.6 m

Answer 1

Answer: (A)

Magnification produced by the lens
$m=\frac{v}{u}=-3\Rightarrow v=-3u$
Applying lens of formula, we have
$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$
$\Rightarrow \frac{1}{-3u}-\frac{1}{u}=\frac{1}{0.12}$
or $\frac{-1-3}{3u}=\frac{100}{12}=\frac{25}{3}$
or $-\frac{4}{3u}=\frac{25}{3}$
or $u=-\frac{4}{25}m=0.16m$