Q. A candle placed 25 $cm$ from a lens, forms an image on a screen placed 75 $cm$ on the other end of the lens. The focal length and type of the lens should be
A) $+ 18.75\ cm$ and convex lens
B) $- 18.75\ cm$ and concave lens
C) $+ 20.25\ cm$ and convex lens
D) $- 20.25\ cm$ and concave lens
Explanation
(a) $u = - 25\ cm,\ v = + 75\ cm$
$\Rightarrow \frac{1}{f} = \frac{1}{+ 75} - \frac{1}{- 25} \Rightarrow f = + 18.75\ cm;$ convex lens
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.