The angle of incidence of a ray to a surface is measured as the difference in angle between the ray and the normal vector of the surface at the point of intersection.
Angle of Incidence
See also
Angle, Angle of AttackThis entry contributed by Wiktor K. Macura
Explore with Wolfram|Alpha
Cite this as:
Macura, Wiktor K. "Angle of Incidence." From MathWorld--A Wolfram Web Resource, created by Eric W. Weisstein. https://mathworld.wolfram.com/AngleofIncidence.html