The angle of refraction measures the angle to which light bends as it passes across the boundary between different media.

- The angle of refraction is measured between a ray of light and an imaginary line called the
*normal*. - In optics, the
*normal*is a line drawn on a ray diagram perpendicular to, so at a right angle to (90^{0}), to the boundary between two media. - If the boundary between the media is curved then the
*normal*is drawn perpendicular to the boundary. - Snell’s law is a formula used to describe the relationship between the angles of incidence and refraction when referring to light passing across the boundary between two different transparent media, such as water, glass, or air.
- In optics, the law is used in ray diagrams to compute the angles of incidence or refraction, and in experimental optics to find the refractive index of a medium.