Angle of incidence (optics)

In computer graphics and geography, the angle of incidence is also known as the illumination angle of a surface with a light source, such as the Earth's surface and the Sun. It can also be equivalently described as the angle between the tangent plane of the surface and another plane at right angles to the light rays. This means that the illumination angle of a certain point on Earth's surface is 0° if the Sun is precisely overhead and that it is 90° at sunset or sunrise.

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. (See specular reflection.)

Grazing angle or glancing angle

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.