Q. A convex lens is in contact with concave lens. The magnitude of the ratio of their focal length is 2/3. Their equivalent focal length is 30 cm. What are their individual focal lengths?
A) -75, 50
B) -10, 15
C) 75, 50
D) -15, 10
Explanation
(d) Let focal length of convex lens is +$f$ then focal length of concave lens would be $- \frac{3}{2}f$.
From the given condition,
$\frac{1}{30} = \frac{1}{f} - \frac{2}{3f} = \frac{1}{3f}$
$\therefore\ f = 10$ cm
Therefore, focal length of convex lens = + 10 cm and that of concave lens = -15 cm.
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.