1. The angle between a ray falling on a surface and the perpendicular (normal) to the surface at the point at which the ray strikes the surface.
2. The angle between a wavefront and a surface that it strikes.
Geology and Earth Sciences
The angle between an incident ray or wave and the normal to a reflecting or refracting surface. The angle of incidence is related to the angle of refraction by Snell’s law. See alsocritical angle.