Question

A convex lens is in contact with concave lens. The magnitude of the ratio of their focal length is of the ratio of their focal length is $\frac{3}{2}$ 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

Answer 1

Answer: (D)

-15, 10

Answer 2

Let focal length of convex lens is + f then focal length of
concave lens would be $-\frac{3}{2} f.$
From the given condition.
$\frac{1}{30}=\frac{1}{f}-\frac{2}{3 f}=\frac{1}{3 f}$
\therefore \hspace15mm f=10 cm
Therefore, focal length of convex lens = + 10 cm
and that of concave lens =- 15 cm
$\therefore$ Correct answer is (d).