Two identical glass μ g = 3 2 equiconvex lenses of focal length, f are kept in contact. The space between the two lenses is filled with water μ w = 4 3 . The focal length of the combination is

Step-by-step solution

1 f = μ g - 1 2 R = 1 R ,   μ g = 3 2 ⇒ R = f 1 f 1 = - μ w - 1 2 R = - 2 3 R = - 2 3 f . 1 f eq = 1 f + 1 f + 1 f 1 = 1 R + 1 R - 2 3 R 1 f eq = 1 f + 1 f - 2 3 f 1 f eq = 2 f - 2 3 f 1 f eq = 4 3 f f eq = 3 f 4 .