Refraction describes the change of direction light experiences when it passes from one medium to another. When a ray of light encounters an interface between two media, two things generally happen. First, some of the light is reflected off the interface (the Law of Reflection applies). Second, some of the light is refracted into the second medium.

The angle of refraction θ₂ (the angle in the second medium) is related to the angle of incidence θ₁ by Snell's Law:

n₁ sin(θ₁) = n₂ sin(θ₂)

where n₁ is the index of refraction of the first medium and n₂ is the index of refraction of the second medium. Angles are measured from the normal to the interface, the line perpendicular to the interface.

When light crosses an interface into a medium with a higher index of refraction, the light bends towards the normal. Conversely, light traveling across an interface from higher n to lower n will bend away from the normal.