Two convex lenses of equal focal lengths are kept in contact with each other. If the power of their combination is 20 D, find the focal length of each convex lens.

Explanation

Data : Convex lens , P=20D, `F_(1)=f_(2)`= ?
The focal length `(f)` of the combinatio of the lenses is given by
`1/f=1/(f_(1))+(1)/(f_(1)) =(2)/(f_(1))`
But `P=(1)/(f) therefore P=(2)/(f_(1))`
`therefore 20D=(2)/(f_(1)) therefore f_(1)=2/(20)m=0.1 m`
`f_(1)=0.1` m
The gives the focal length of each convex lens.