Q. An object has image thrice of its original size when kept at $8\ cm$ and $16\ cm$ from a convex lens. Focal length of the lens is
A) 8 $cm$
B) 16 $cm$
C) Between 8 $cm$ and $16\ cm$
D) Less than 8 $cm$
Explanation
(c) $m = \pm 3$, using $m = \frac{f}{f + u}$
For virtual image $3 = \frac{f}{f - 8}$ …(i)
For real image $- 3 = \frac{f}{f - 16}$ …(ii)
Solving (i) and (ii) we get $f = 12\ cm$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.