Question

How does the angle of minimum deviation of a glass prism vary, if the incident violet light is replaced by red light? Give reason.

Answer 1

Refractive index of the prism (μ) is inversely proportional to the wavelength of the incident light (λ) i.e.
μ ∝ 1/λ    ...(i)
Also, minimum deviation(δm) is related to the refractive index as: 

δ_m ∝ μ

from (i)

δ_m ∝ 1/λ  ...(ii)

As we know wavelength of red light is more than the violet light. Therefore, using (ii), we can conclude that the angle of minimum deviation for red light is less than the violet light.

Answer 2

The angle of minimum deviation is given by δ_m  = ( n₂₁ - 2 ) A 
If voilet light light is replaced by red light its refractive index decreses and angle of minimum deviation  decreases.