Snell's law
The fundamental law that governs the refraction of light is Snell's Law. According to Snell's Law, when a light ray is transmitted into a new medium, the relationship between the angle of incidence (θi) and the angle of refraction (θr) is given by the following equation
ni sin(θi) = nr sin(θr)
where the ni and nr values represent the refractive indexes (which are unitless) of the incident and the refractive medium respectively.
Figure 1: The incident and the refractive angles according to Snell's law.