Two lenses are placed in contact with each other and the focal length of combination is `80 cm`. If the focal length of one is `20 cm`, then the power of the other will be

Explanation

Here, `F = 80 cm, f_1 = 20 cm, P_2 = ?`
`P = (100)/(F) = (100)/(80) = 1.25 D, P_1 = (100)/(f_1) = (100)/(20) = 5 D`
As `P_1 + P_2 = P :. P_2 = P - P_1 = 1.25 - 5 = -3.75 D`.