Two lenses 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) = -5D`
`P = P_(1) + P_(2) = 15 - 5 = 10 D`
`F = (100)/(P) = (100)/(10) = 10cm`
Now, `u = - 30 cm, v = ?`
from `(1)/(v) = (1)/(F) + (1)/(u) = (1)/(10) - (1)/(30) = (1)/(15)`
`v = 15 cm`