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The Fresnel equations (or Fresnel coefficients) describe the reflection and transmission of light (or electromagnetic radiation in general) when incident on an interface between different optical media.
The reflection of light from a single interface between two media is described by the Fresnel equations. However, when there are multiple interfaces , such as in the figure, the reflections themselves are also partially transmitted and then partially reflected.
Reflectivity is the square of the magnitude of the Fresnel reflection coefficient, [4] which is the ratio of the reflected to incident electric field; [5] as such the reflection coefficient can be expressed as a complex number as determined by the Fresnel equations for a single layer, whereas the reflectance is always a positive real number.
In 3D computer graphics, Schlick’s approximation, named after Christophe Schlick, is a formula for approximating the contribution of the Fresnel factor in the specular reflection of light from a non-conducting interface (surface) between two media.
A Fresnel lantern with the lens open to show the ridges. Glass Fresnel lenses also are used in lighting instruments for theatre and motion pictures (see Fresnel lantern); such instruments are often called simply Fresnels. The entire instrument consists of a metal housing, a reflector, a lamp assembly, and a Fresnel lens.
The fundamental principle behind the operation of an FBG is Fresnel reflection, where light traveling between media of different refractive indices may both reflect and refract at the interface. The refractive index will typically alternate over a defined length.
Thus, whatever phase is associated with reflection on one side of the interface, it is 180 degrees different on the other side of the interface. For example, if r has a phase of 0, r’ has a phase of 180 degrees. Explicit values for the transmission and reflection coefficients are provided by the Fresnel equations
Augustin-Jean Fresnel [Note 1] (10 May 1788 – 14 July 1827) was a French civil engineer and physicist whose research in optics led to the almost unanimous acceptance of the wave theory of light, excluding any remnant of Newton's corpuscular theory, from the late 1830s [3] until the end of the 19th century.