Incidence

Part of speech: noun (n.)

1. A falling on or upon; an incident; an event. [Obs.] Bp. Hall.

2. The direction in which a body, or a ray of light or heat, falls on any surface. In equal incidences there is a considerable inequality of refractions. Sir I. Newton. Angle of incidence, the angle which a ray of light, or the line of incidence of a body, falling on any surface, makes with a perpendicular to that surface; also formerly, the complement of this angle. -- Line of incidence, the line in the direction of which a surface is struck by a body, ray of light, and the like.