What is "line of incidence"

Incidence \In"ci*dence\, n. [Cf. F. incidence.]

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

2. (Physics) 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.

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.

   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.