Q. A person can see clearly only upto a distance of $25\ cm$. He wants to read a book placed at a distance of $50\ cm$. What kind of lens does he require for his spectacles and what must be its power
A) Concave, $- 1.0\ D$
B) Convex, $+ 1.5\ D$
C) Concave, $- 2.0\ D$
D) Convex, $+ 2.0\ D$
Explanation
(c) For correcting myopia, concave lens is used and for lens.
$u =$ wants to see $= - 50\ cm$
$v =$ can see $= - 25\ cm$
From $\frac{1}{f} = \frac{1}{v} - \frac{1}{u} \Rightarrow \frac{1}{f} = \frac{1}{- 25} - \frac{1}{( - 50)} \Rightarrow f = - 50\ cm$
So power $P = \frac{100}{f} = \frac{100}{- 50} = - 2D$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.