A method of computing the propagation of waves where the waves are treated as having very small wavelengths. A series of rays are allowed to propagate from a source or sources in straight lines. If a ray encounters the surface of an object it is reflected from the surface and its new path followed. From a large set of such calculations the effects of the sources can be derived. Geometric optics is often used as a technique for the construction of realistic images for three-dimensional computer graphics. It becomes less reliable as the wavelength of the wave under consideration becomes larger. See also geometric theory of diffraction.