Angle of incidence ( optics ) - wikipedia Angle of incidence ( optics ) In geometric optics, the angle of incidence 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. The ray can be formed by any wave : optical, acoustic, microwave, X-ray and so on. In the figure below, the line representing a ray makes an angle θ with the normal ( dotted line ). The angle of incidence at which light is first totally internally reflected is known as the critical angle. The angle of reflection and angle of refraction are other angles related to beams. 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. Refraction of light at the interface between two media. Contents 1 Glancing angle 2 See also 3 Notes 4 External links Glancing angle 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. See also Effect of sun angle on climate Reflection ( physics ) Refraction Season Total internal reflection Notes Jump up ^ Allen R Miller and Emanuel Vegh ( 1993 ). `` Exact Result for the Grazing Angle of Specular Reflection from a Sphere ''. SIAM Review. 35 : 472 -- 480. doi : 10.1137 / 1035091. External links Weisstein, Eric W. `` Angle of incidence ''. MathWorld. geometry : rebound on the strip billiards Flash animation Retrieved from `` https://en.wikipedia.org/w/index.php?title=Angle_of_incidence_(optics)&oldid=819752746 '' Categories : Geometrical optics Angle Talk Contents About Wikipedia Català Dansk Deutsch Español Français 한국어 Italiano မြန်မာဘာသာ Norsk nynorsk Polski Português Svenska 中文 4 more Edit links This page was last edited on 11 January 2018, at 02 : 34. Text is available under the Creative Commons Attribution - ShareAlike License ; additional terms may apply. By using this site, you agree to the Terms of Use and Privacy Policy. Wikipedia ® is a registered trademark of the Wikimedia Foundation, Inc., a non-profit organization. About Wikipedia

Question: where is the angle of incidence measured from
Answer: between a ray incident on a surface and the line perpendicular to the surface at the point of incidence, called the normal