Question

A convex lens is in contact with a concave lens. The magnitude of the ratio of their focal lengths is 23 . Their equivalent focal length is 30 cm. What are their individual focal lengths?(in cm)

• A. –75, 50
• B. -10, 15
• C. –75, 25
• D. –15, 10

Answer 1

The correct option is D –15, 10

Let focal length of convex lens is +f, then length of concave lens would be – 32f.
From the given condition,
130=1f−23f=13f
∴ f = 10
Therefore, focal length of convex lens = +10 cm
And that of concave lens = –15 cm.