Two lenses, one diverging of power 2D and the other converging of power 6D are combined together. Calculate the focal length and power of the combination.

Explanation

`P_(1)=-2D`
`P_(2)=+6D`
Power of combination of lenses `=P=P_(1)+P_(2)`
`P=2+6=-4D`
Let `f=` Focal length of combination of lenses
`impliesP=1/(f("in m"))`
`impliesf=1/P=1/4=0.25m`
`impliesf=25cm`