Incidence /(?)/

In·ci·dence

Incidence

n.
  1. A falling on or upon; an incident; an event; an occurrence. [Obs.]
  2. The direction in which a body, or a ray of light or heat, falls on any surface. (Physics)
    In equal incidences there is a considerable inequality of refractions.
  3. The rate or ratio at which something occurs; as, the incidence of murder in Los Angeles; the incidence of cancer in men over 50.

Phrases & Compounds
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.