Question

What is the focal length of a convex lens of focal length 30 cm in contact with a concave lens of focal length 20 cm? Is the system a converging or a diverging lens? Ignore thickness of the lenses.

Answer 1

Focal length of the convex lens, f₁ = 30 cm 

Focal length of the concave lens, f₂ = −20 cm 

Focal length of the system of lenses = f 

The equivalent focal length of a system of two lenses in contact is given as:

Hence, the focal length of the combination of lenses is 60 cm. The negative sign indicates that the system of lenses acts as a diverging lens.