What is atmospheric refraction??

Refractive index (n) is a dimensionless quantity that has no units. Since both numerator and denominator are in meters per seconds (m/s), both units cancel each other. This makes refractive index simply a number. 

Absolute refractive index is the ratio of speed of light in air/vacuum to speed of light in any medium. It is closely related to refractive index, except that it is relative to vacuum and not to any other medium. This is different from relative refractive index because it is the ratio of speed of light in one medium compared to the speed of light in another medium.

The refractive index is an optical property of the material that describes how much does the light bend or refract while passing from one medium to another. It is the ratio of spee of light in vacuum (c) to speed of light in a material (v)$n=\frac{c}{v}$ Refractive index is the reason which causes refraction. When light travels from a rarer medium (low refractive index) to denser medium (higher refractive index), it will slow down and bend toward the normal. Refractive index also changes according to the wavelength of light. Air has 1.003 refractive index, water has 1.33 refractive index and glass has a refractive index of 1.5 to 1.9.

μ? sin 30° = μ? sin θ

⇒ 2.42 × (1/2) = (1) sinθ
sinθ = 1.21 > 1
Refraction is not possible, total internal reflection

Rainbow is formed due to internal reflection and dispersion.