Snell’s law

The law of refraction: the refracted ray lies in the plane of incidence.

Snell’ law is

n₁ sin θ₁ = n₂ sin θ₂

Where

n₁ and n₂ are the index of refraction of medium 1 and medium 2.
The index of refraction of a medium is the ratio between the speed of light c in vacuum and the speed of light v in that medium

n = c/v


θ₁, θ₂ are angles of incidence and angle of refraction in the medium 1 and medium 2.

(Chapter: Light waves)