# Fresnel reflection loss

**Fresnel reflection, Fresnel equations ,Fresnel coefficients**

The returning part of the incident light at a plane surface between two homogeneous and isotropic media with different refractive indices is called Fresnel reflection loss. This can be calculated (as can transmission) using the Fresnel equations. The ratio of the reflected or transmitted amplitude to the amplitude of the incoming wave is called the reflection and transmission coefficients. When considering energy quantities, the respective intensity ratios are described by the degree of reflection or the degree of transmission. For example, in fiber optics, there is an air gap (glass/air gap/glass) at each connector because the polished optical fiber surfaces of the connectors have minimal roughness that creates an air gap, or because the optical fibers have different refractive indices. These interfaces represent discontinuities of refractive index (refractive index discontinuities) where losses occur, and of course this also applies to fiber end faces. At all these discontinuities a part of the optical signal is reflected back to the source (Fresnel reflection), which can be characterized and localized e.g. by OTDR measurement techniques.