Angle of incidence

The angle between an incoming ray or wave and the normal to the surface it strikes. It is measured from the normal, not from the surface itself.

Snell's law: The equation relating the angles and refractive indices of waves crossing a boundary. It uses the incidence angle to calculate a transmitted ray’s direction.
Surface normal: A line perpendicular to a surface at a specified point. The incidence angle is measured from this line at the point of contact.
Specular reflection: Reflection from a smooth surface in which parallel incoming rays remain parallel after reflection. Its geometric rule equates the reflected and incident angles measured from the normal.
Angle of attack: The angle between an aircraft wing’s reference chord line and the oncoming airflow. It is measured against a body-fixed line, unlike incidence angle’s surface normal.
Fresnel equations: Equations describing how much light is reflected and transmitted at an interface. They predict boundary reflection from incidence angle, polarization, and refractive indices.
Angle of reflection: The angle between a reflected ray and the surface normal. For specular reflection, it equals the angle of incidence.
Diffuse reflection: Reflection from a rough surface that scatters incoming light across many directions. Surface irregularities prevent one incidence angle from determining a single reflected direction.
Angle of elevation: The angle between a line of sight and the horizontal plane, measured upward. It uses the horizon as its reference rather than a surface normal.
Brewster's angle: The incidence angle at which reflected light with one polarization vanishes at a dielectric interface. It is a special incidence angle that produces completely polarized reflected light.
Angle of refraction: The angle between a transmitted ray and the surface normal. It is paired with the incidence angle in refraction laws.

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