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Snell's law

The basic physics of operation for any planar electromagnetic absorber involves a fundamental concept defined by Snell's law. When an electromagnetic wave travelling through free-space encounters a different medium the wave will be reflected transmitted and absorbed. It is of course the magnitude of the reflected signal which is usually of interest in the case of an electromagnetic absorber. Reflectivity is the ratio between intensity of reflected wave on incident wave and defined performances of anechoic chambers.

θ_i θ_r : Incident and reflected angles of the incident and reflected fields which are equal.
Eⁱ Hⁱ : Electric and Magnetic incident fields
E^t H^t : Electric and  Magnetic transmit fields
E^r H^r : Electric and Magnetic reflected fields

If we define the wave impedance of a material as Z