Angle of incidence (optics)

It is sometimes more useful to refer to the angle between the beam and the surface tangent, rather than that between the beam and the surface normal. The 90-degree complement to the angle of incidence is called the grazing angle or glancing angle. For example, a ray with 30° angle of incidence has a 60° grazing angle. A beam or ray that is nearly parallel to a surface (i.e., having small grazing angles) is called a "grazing beam" or a "grazing ray". For example, 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. Similarly, a Wolter telescope used for X-ray astronomy is based on the principle of total external reflection at small grazing angles. Moreover, ridged mirrors are designed to reflect atoms coming at a small grazing angle, usually measured in milliradians. In optics, there is Lloyd's mirror. ==See also==