Question

A convex lens is in contact with a concave lens. The magnitude of the ratio of their focal lengths is 2/3. Their equivalent focal length is 30 cm. What are their individual focal lengths?

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

Answer 1

The correct option is A -15, 10

Let focal length of convex lens be $f$

Then focal length of the concave lens $=-1.5f$

Equivalent focal length $=\frac{-1.5f\times f}{(-0.5f)}=3f$

=>$f=10cm$

So individual focal lengths are 10 cm and -15 cm