Question

A converging lens of focal length 40 cm is kept in contact with a diverging lens of focal length 30 cm. Find the focal length of the combination .

Answer 1

`1/(F') = 1/(f_1) + 1/(f_2) = 1/(+40) +1/(-30) =1/40 - 1/30`

`1/(F') = (3-47)/120 = 1/120 `cm  [Diverging]