Q. A person who can see things most clearly at a distance of $10\ cm$. Requires spectacles to enable to him to see clearly things at a distance of $30\ cm$. What should be the focal length of the spectacles
A) $15\ cm$ (Concave)
B) 15 $cm$ (Convex)
C) $10\ cm$
D) 0
Explanation
(a) For lens $u =$ wants to see $= - 30\ cm$
And $v =$ cab see $= - 10\ cm$
$\therefore\frac{1}{f} = \frac{1}{v} - \frac{1}{u} = \frac{1}{- 10} - \frac{1}{( - 30)} \Rightarrow f = - 15\ cm$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.