The law of refraction: the refracted ray lies in the plane of incidence.
Snell’ law is
n1 sin θ1 = n2 sin θ2
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)