Relative refractive index of medium 2 with respect to medium 1:

₁μ₂ = μ₂ / μ₁

where μ₂ and μ₁ are absolute refractive indices of medium 2 and medium 1.

Factors on which relative refractive index depends:

1. Nature of the two media.
2. Wavelength (or colour) of the light.

OR

Let the angle of prism A = 60°.

For minimum deviation, the light ray passes symmetrically, i.e. angle

of incidence i = angle of emergence e.

So, angle of refraction r = A/2 = 30°.

Relation:

δmin = 2i – A

Since i = r when symmetrical,

i = A/2 + δmin/2.

Thus, δmin occurs when the ray passes symmetrically through the prism.