A convex lens of focal length `f_(1)` is kept in contact with a concave lens of focal length `f_(2)`. Find the focal length of the combination.

Explanation

For convex lens, focal length, `f = f_(1)` and for concave lens, the focal length, `f = -f_(2)`.
The equivalent focal length of a combination of convex lens and concave lens is given as :
`(1)/(F)=(1)/(f_(1))+(1)/(-f_(2))`
`F = (f_(1)f_(2))/(f_(2)-f_(1))`.