Question

A converging lens is kept co-axially in contact with a diverging lens – both the lenses being of equal focal lengths. What is the focal length of the combination?

Answer 1

Let focal length of converging and diverging lenses be + f and - f respectively.

Power of converging len P₁ = 1/f, Power of diverging lens P₂ = - 1/f

∴ Power of combination P = P₁ + P₂ = 1/f - 1/f = 0

∴ Focal length of combination F 1/P = 1/0 = ∞ (infinite)​

F = ∞.