\In"ci*dence\, n. [Cf. F. incidence.]
1. A falling on or upon; an incident; an event. [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.
{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.