A convex lens of focal length `25 cm` is placed co-axially in contact with a concave lens of focal length `20 cm`. Determine the power of the combination. Will the system be converging or diverging in nature ?

Answer

`1 D` ; diverging

Explanation

Here, `f_(1) = 25 cm, P_(1) = (100)/(f_(1)) = (100)/(25) = 4 D`
`f_(2) = - 20 cm, P_(2) = (100)/(f_(2)) = (100)/(- 20) = - 5 D`
`P = P_(1) + P_(2) = 4 - 5 = - 1 D`, diverging.