The thickness at the centre of a plano convex lens is 3 mm and the diameter is 6 cm . If the speed of light in the material of the lens is 2 × 10 8 m s - 1 . The focal length of the lens is ___________.

Step-by-step solution

R 2 = r 2 + ( R - t ) 2 R 2 = r 2 + R 2 + t 2 - 2 R t Neglecting t 2 , we get R = r 2 2 t ∴ 1 f = μ - 1 1 R - 1 ∞ = μ - 1 R f = R μ - 1 = r 2 2 t ( μ - 1 ) = 3 × 10 - 2 2 2 × 3 × 10 - 3 × 3 2 - 1 = 9 × 10 - 4 6 × 10 - 3 × 1 × 2 f = 0 . 3   m = 30   cm