A ray of light travelling through rarer medium is incident at a very small angle 'i' on a glass slab and after refraction its velocity is reduced by 20 % . The angle of deviation is

Step-by-step solution

Let the velocity of light in air be v₁ and in glass be v₂ , with corresponding refractive indices n₁ = 1 and n₂ . The velocity decreases by 20%, so v₂ = 0.8v₁ . Using the relation n 1/v , we have n₁ n₂ = v₂ v₁ = 0.8 . Solving gives n₂ = 1 0.8 = 5 4 . Snell's law states n₁ i = n₂ r . For small angles, , so i = 5 4 r and r = 4 5 i . The deviation is = i - r = i - 4 5 i = 1 5 i .