Two lens of power `+ 15 D and - 5 D` are in contact with each other. What is the focal length of the combination ? What would be the position of image formed by the combination for an object at `30 cm` ?

Explanation

Here, `P_1 = 15 D, P_2 = -5 D`
`P = P_1 + P_2 = 15 - 5 = 10 D`
`F = (100)/(P) = (100)/(10) = 10 cm`
Now, `u = -30 cm, v = ?`
from `(1)/(v)=(1)/(F)+(1)/(u)=(1)/(10)-(1)/(30)=(1)/(15)`
`v = 15 cm`.