State laws of refraction

The laws of refraction are two rules that describe how light bends when passing from one medium into another.

Refraction is the change in direction of a ray of light when it passes from one transparent medium into another because its speed changes. 

1. The incident ray, the refracted ray and the normal to the boundary all lie in the same plane.
2. At the interface, the ratio of the sines of the angles of incidence and refraction equals the ratio of the phase velocities (or equivalently the inverse ratio of refractive indices) of the two media.

These laws are often called Snell's law in optics problems and help in lens design, fibre optics and many applications.