Two thin lenses are of same focal lengths f , but one is convex and the other one is concave. When they are placed in contact with each other, the equivalent focal length of the combination will be:

Step-by-step solution

The formula to calculate the effective focal length of the combination can be written as 1 f eff = 1 f convex + 1 f concave       . . . 1 Substitute f for f convex and - f for f concave into equation (1) and solve to obtain the effective focal length of the combination. 1 f eff = 1 f - 1 f = 0 ⇒ f eff = ∞