This geometric measurement describes how light bounces off a boundary. It is measured from the normal line, which is perpendicular to the surface at the exact point of contact, to the path of the departing ray. The law of reflection dictates that this angle will always perfectly match the angle of incidence when light hits a smooth, specular surface like a mirror.
Calculate the angle of reflection so we can position the secondary mirror exactly where the beam exits the primary.
Always measure from the normal, not from the surface itself. Measuring from the surface is a common rookie mistake that throws off calculations.
Related terms
Angle of incidence
The angle between an incoming light ray and the normal line perpendicular to the surface it strikes.
Angle of refraction
The angle between a refracted light ray and the normal line as it passes through a new medium.
Aspecular
Measured at an angle offset from the direct angle of reflection.