Question

The distance at which an object should be placed in front of a convex lens of focal length 10 cm to obtain a real image double the size of object will be:

• A. 30 cm
• B. 15 cm
• C. 5 cm
• D. 10 cm

Answer 1

Answer: (B)

15 cm

Convex lens gives the real and double-sized image when the object is placed exactly between the focus and radius of curvature.
We have, $\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$
$m=\frac{v}{u}=2$ or $v=2u$

$\therefore \frac{1}{f}=\frac{1}{2u}-\frac{1}{-u}=\frac{1}{2u}+\frac{1}{u}=\frac{3}{2u}$

or $\frac{1}{10}=\frac{3}{2u}$ or $u=15cm$