<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> <P> 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". </P> <P> 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 . </P>

Angles in optics are measured with respect to the surface