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 convex lens, f₁ = 30 cm.

focal length of convex lens, f₂ = –20 cm.

focal length of the combination,

1/f = 1/f₁ + 1/f₂ = 1/30 + 1/-20 = (2 - 3)/60 = -1/60

∴ f = - 60cm.

The system behaves as a diverging lens.