line of incidence

Incidence \In"ci*dence\, n. [Cf. F. incidence.]
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   1. A falling on or upon; an incident; an event; an
      occurrence. [Obs.] --Bp. Hall.
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   2. (Physics) The direction in which a body, or a ray of light
      or heat, falls on any surface.
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            In equal incidences there is a considerable
            inequality of refractions.            --Sir I.
                                                  Newton.
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   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.
      [PJC]

   {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.
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