Diameter of a plano - convex lens is 6 cm and thickness at the centre is 3 mm . If the speed of light in the material of lens is 2 × 1 0 8 m s - 1 , the focal length of the lens is:

Step-by-step solution

A B = 6   cm r = 3   cm O A 2 = O P 2 + P A 2 R 2 = R - t 2 + r 2 R 2 = R 2 + t 2 - 2 R t + r 2 2 R t = t 2 + r 2 R = t 2 + r 2 2 t = 0 · 3 2 + 3 2 2 × 0 · 3 R ≈ r 2 2 t = 3 2 2 × 0 · 3 = 9 0 · 6 = 9 0 6 = 1 5   cm Use 1 f = μ - 1 1 R 1 - 1 R 2 1 f = 1 . 5 - 1 1 1 5 = 1 3 0 f = 3 0   cm