You are given two convex lenses.
a. What is the focal length of the combined lens formed by the lenses put in contact ?
b . What is the advantage in the combination ?
c. if one of the lenses is concave, how will you find its focal length with the help of lens combination ?

Explanation

`a. (1)/(F) = (1)/(f_(1)) + (1)/(f_(2))`
F - Focal length of combination of lenses
`f_(1)` - Focal length of one lens
`f_(2) -` Focal length of second lens
`f_(2)` - Focal length of second lens
b, i. increases magnifying power .
ii. For erecting an image.
c. the focal length .f. of convex lens and .F. of the lens combination can be found out by u-v method. Let `f_(2)` be the focal length of the concave lens. Then
`(1)/(f_(2)) = (1)/(F) - (1)/(f_(1))`
Knowing `f_(1) and F, .f_(2).` can be found out .