<P> In geometric optics, the angle of incidence is the angle between a ray incident on a surface and the line perpendicular to the surface at the point of incidence, called the normal . The ray can be formed by any wave: optical, acoustic, microwave, X-ray and so on . In the figure below, the 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 . The angle of reflection and angle of refraction are other angles related to beams . </P> <P> Determining the angle of reflection with respect to a planar surface is trivial, but the computation for almost any other surface is significantly more difficult . The exact solution for a sphere (which has important applications in astronomy and computer graphics) was an open problem for nearly 50 years until a closed - form result was derived by mathematicians Allen R Miller and Emanuel Vegh in 1991 . </P>

Where is the angle of incidence measured from