If the convex lens of focal length 10 cm and a concave lens of focal length 50 cm are kept in contact with each other.
What will be the power of combination ?

Explanation

Data :`f_(1)=+10 cm=+0.1 m` (convex lens)
`f_(2)=-50 cm=-0.5 m ` (concave lens),
`f`(combination)= ? ,P(combination)= ?
`P_(1)=(1)/(f_(1))=(1)/(0.1 m)=D`
`P_(2)=(1)/(f_(2))=(1)/(-0.5m)=-2D`
`therefore P=P_(1)+P_(2)=10D-2D`
=8 D
`" "`OR
`P=1/f =(1)/(0.125 m)=8D`
The power of the combination of the lenses= 8 D.