Q. A magnifying glass is to be used at the fixed object distance of 1 $inch$. If it is to produce an erect image magnified $5\ times$ its focal length should be
A) $0.2\ inch$
B) $0.8\ inch$
C) $1.25\ inch$
D) $5\ inch$
Explanation
(c) $m = \frac{v}{u} = 5 \Rightarrow v = 5\ inch\ \lbrack Given\ u = 1\ inch\rbrack$
Using sign convention $u = - 1\ inch,\ v = - 5\ inch$
$\therefore\frac{1}{f} = \frac{1}{v} - \frac{1}{u} = \frac{1}{- 5} - \frac{1}{- 1} \Rightarrow f = 1.25\ inch$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.