Two lenses of power `-1.75D and +2.25 D` respectively, are placed in contact. Calculate the focal length of the combination.

Explanation

`P_(1)=-1.75D,P_(2)=+2.25D.`
`P=P_(1)+P_(2)`
`P=-1.75+2.25`
P = 0.5
`(1)/(F)=P`
`F=(1)/(P)=(1)/(0.5)=2m`
F = 200 cm