Question

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 (in cm)?

• A. –75, +50
• B. +10, –15
• C. +75, –50
• D. –15, –10

Answer 1

The correct option is B

+10, –15

$\frac{1}{f_{eff}}=\frac{1}{f_1}+\frac{1}{f_2}Put{f}_1=2f{f}_2=-3f\text{s}olving,f=5cm$