From Wikipedia, the free encyclopedia
The angle of incidence is the angle which references the sun’s radiation striking a surface. It 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.
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.
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 that between the beam and the surface normal, in other words 90° minus the angle of incidence. This small angle is called a glancing angle or grazing angle. Incidence at grazing angles is called “grazing incidence”.
Grazing incidence diffraction is used in X-ray spectroscopy and atom optics, where significant reflection can be achieved only at small values of the grazing angle. Ridged mirrors are designed for reflection of atoms coming at small grazing angle. This angle is usually measured in milliradians. In optics, there is Lloyd’s mirror.