Fresnel equations
The Fresnel equations (or Fresnel conditions), deduced by Augustin-Jean Fresnel (/freɪˈnɛl/), describe the behaviour of light when moving between media of differing refractive indices. The reflection of light that the equations predict is known as Fresnel reflection.
When light moves from a medium of a given refractive index, n1, into a second medium with refractive index, n2, both reflection and refraction of the light may occur. The Fresnel equations describe what fraction of the light is reflected and what fraction is refracted (i.e., transmitted). They also describe the phase shift of the reflected light.
The equations assume the interface between the media is flat and that the media are homogeneous. The incident light is assumed to be a plane wave, and effects of edges are neglected.
The behavior depends on the polarization of the incident ray, which can be separated into 2 cases:
In the diagram on the right, an incident light ray, IO, strikes the interface between two media of refractive indices n1 and n2 at point, O. Part of the ray is reflected as ray, OR, and part refracted as ray, OT. The angles that the incident, reflected and refracted rays make to the normal of the interface are given as θi, θr and θt, respectively.
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