Q. A simple magnifying lens is used in such a way that an image is formed at $25\ cm$ away from the eye. In order to have $10\ times$ magnification, the focal length of the lens should be
A) $5\ cm$
B) $2\ cm$
C) $25\ mm$
D) $0.1\ mm$
Explanation
(c) $m = 1 + \frac{D}{f_{e}} \Rightarrow 10 = 1 + \frac{25}{f_{e}} \Rightarrow f_{e} = \frac{25}{9} \simeq 25\ mm$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.