At what distance should an object be placed from a convex lens of focal length 15 cm to obtain a real image three times the size of the object?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 20 cm
Magnification formula, $m=\frac{v}{u}=-3$ [Given]
$⇒v=-3u$
using lens formula,
$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$
$\frac{1}{15}=\frac{1}{-3u}-\frac{1}{u}$
$⇒u=-\frac{4×15}{3}=-20cm$