The angle of incidence measures the angle at which incoming light strikes a surface.

- The angle of incidence is measured between a ray of incoming 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}), the boundary between two media. - If the boundary between the media is curved, then the normal is drawn at a tangent to the boundary.