# Angle of incidence

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

###### About lines that are normal to one another
• If one line is normal to another, then it is at right angles.
• In geometry, a normal (or the normal) refers to a line drawn perpendicular to and intersecting another line, plane or surface.
• In the field of optics, the normal is a line drawn on a ray diagram perpendicular to (at 900 to), the boundary between two media.
• If the boundary between two media is curved then the normal is drawn at a tangent to the boundary.
###### References

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 (900), the boundary between two media.
• If the boundary between the media is curved, then the normal is drawn at a tangent to the boundary.