Question

A convex lens of focal length f₁ is kept in contact with a concave lens of focal length f₂. Find the focal length of the combination. 

Answer 1

For convex lens, focal length, f = f₁ and for concave lens, the focal length, f = -f₂

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_1f_2)/(f_2 -f_1)`