Two identical thin biconvex lenses of focal length 15 cm and refractive index 1 . 5 are in contact with each other. The space between the lenses is filled with a liquid of refractive index 1 . 25 . The focal length of the combination is _____ cm .

Step-by-step solution

For convex lenses, let radius of curvature is R . Then, 1 15   cm = 1 . 5 - 1 1 R - 1 - R ⇒ 1 R = 1 15   ⇒ R = 15   cm Now for concave lens, 1 f c o n c a v e = 1 . 25 - 1 1 - R - 1 R = 1 4 × - 2 R = - 1 30 1 f e f f = 1 15 + 1 15 - 1 30 = - 1 10 ⇒ f e f f = - 10   cm