Two identical thin Plano-convex glass lenses (refractive index 1 .5 ) each having radius of curvature of 20 cm are placed with their convex surfaces in contact at the center. The intervening space is filled with oil of refractive index 1 .7 The focal length of the combination is

Step-by-step solution

Len's maker's formula 1 f = μ - 1 1 R 1 - 1 R 2 1 f 1 = μ - 1 1 20 ⇒ 1.5 - 1 1 20 = 1 40 1 f 1 = 1 40 For lens 1 f 1 + 1 f 2 = 1 40 + 1 40 = 1 20 For concave 1 f 3 = 1.7 - 1 ( - 1 R - 1 R ) = - 0.7 × 2 R 1 f e q = 1 20 + - 0.7 × 2 R = 1 20 - 1.4 20 f e q = - 20 0.4 = - 50 c m