A convex lens of focal length 20 cm is placed in contact with a concave lens of focal length 10 cm. What is the focal length and power of the combination ?

Explanation

Here, `f_(1) = + 20 cm, f_(2) -10 cm, F = ?, P = ?`
From `(1)/(f) = 1/(f_(1)) + 1/(f_(2)) = 1/20 - 1/10 = -1/20, F = -20 cm`.
`P = 100/F = 100/(-20) = -5` dioptre.