Two thin lenses of power +5 D and -2.5 D are in contact. What is the focal length of the combination ?

Explanation

Given, focal length of the first lens, `f_(1)=+10cm`
`=0.10 m`
Focal length of second lens, `f_(2)=-5cm =-0.05m`
(i) Focal length of combination is givne by
`(1)/(f)=(1)/(f_(1))+(1)/(f_(2))=(1)/(0.10)+((-1)/(0.05))=-10`
`rArr" "f=(-1)/(10)m=-0.10m=-10cm`
(ii) Power of the combination is
`P=P_(1)+P_(2)=(1)/(f_(1))+(1)/(f_(2))=-10D`