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?
Answer & explanation
Correct answer: option 4
$\frac{f_1}{f_2}=\frac{2}{3}$ (1)
$\frac{1}{f_1}-\frac{1}{f_2}=\frac{1}{30}$ (2)
Solving equations (1) and (2),
⇒ f2 = 15 cm (concave lens)
f1 = 10 cm (convex lens)
∴ (d)