A convex lens of focal length ‘f’ is placed in contact with a concave lens of the same focal length. The equivalent focal length of the combination is

Step-by-step solution

Given, focal length of convex lens, f 1 = f and focal length of concave lens, f 2 = - f . The focal length of combination, when two lenses are in contact is given by 1 f = 1 f 1 + 1 f 2 Substituting given value in Eq. (i). We get ⇒ 1 f = 1 f - 1 f = 0 ⇒ f = ∞ Hence, the equivalent focal length of the combination is infinity.