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.