Q. A converging beam of rays is incident on a diverging lens. Having passed through the lens the rays intersect at a point $15\ cm$ from the lens on the opposite side. If the lens is removed the point where the rays meet will move $5\ cm$ closer to the lens. The focal length of the lens is
A) $- 30\ cm$
B) $5\ cm$
C) $- 10\ cm$
D) $20\ cm$
Explanation
(a) $\frac{1}{v} - \frac{1}{u} = \frac{1}{f}$
$u = 10,\ v = 15,\ f = ?$
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$\frac{1}{15} - \frac{1}{10} = \frac{1}{f}$
$\Rightarrow f = - 30\ cm$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.