Q. A person is suffering from myopic defect. He is able to see clear objects placed at $15\ cm$. What type and of what focal length of lens he should use to see clearly the object placed $60\ cm$ away
A) Concave lens of $20\ cm$ focal length
B) Convex lens of $20\ cm$ focal length
C) Concave lens of $12\ cm$ focal length
D) Convex lens of $12\ cm$ focal length
Explanation
(a) For viewing far objects, concave lenses are used and for concave lens
$u =$ wants to see $= - 60cm;v =$ can see $= - 15\ cm$ so from $\frac{1}{f} = \frac{1}{v} - \frac{1}{u} \Rightarrow f = - 20\ cm$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.