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An angle of incidence is the angle between a beam incident on a surface and the line perpendicular to the surface at the point of incidence, called the normal. The beam can be formed by any wave: optical, acoustical, microwave, X-ray etc. In Fig. 1 the red line representing a ray makes an angle θ with the normal (dotted line). The angle of incidence at which light is first totally internally reflected is known as the critical angle.
Alternate terminology
When dealing with a beam that is nearly parallel to a surface, it is sometimes more useful to refer to the angle between the beam and the surface, rather than the beam and the surface normal. This angle is called a glancing angle or grazing angle. It is used in such applications as X-ray spectroscopy.
Aviation
Another common usage is in aviation, where it refers to the angle between the wing's chord and the longitudinal axis of an aircraft (a fixed value). Fig. 2 shows a side view of part of an aeroplane. The wing (dotted blue line) makes an angle a with the fuselage (solid blue line). The wings are typically mounted at a small positive angle of incidence, to allow the fuselage to be "flat" to the airflow in normal cruising flight. Angles of incidence of about 6° are common on most general aviation designs.
Another term for angle of incidence in this context is rigging angle. It should not be confused with the angle of attack, which is the angle the wing chord presents to the airflow in flight. Note that some ambiguity in this terminology exists, as some engineering texts that focus solely on the study of airfoils and their medium may use either term when referring to angle of attack.
See also
"Angle of incidence"