If a light ray is incident at an angle then it shows minimum deviation, also then
2. Refracted ray travels parallel to base of the prism if the prism is equilateral
3. δmin. = 2 i – A = 2 i – 2 r = 2 α
An equilateral prism (i.e. prism angle A = 600) made of ordinary glass (i.e. μ = 1.5) & placed in air (i.e. μ1 = 1), at incidence angle of the 490 shows minimum deviation of 380 with refraction angle of 300.