Fresnel equations specify the amplitude coefficients for transmission and reflection at the interface between two transparent homogeneous media:
For example, ts is the amplitude transmission coefficient for s polarization; the transmitted amplitude is that factor times the incident amplitude in that case (disregarding any phase changes for transmission in the media). n1 and n2 are the refractive indices of the two media. The corresponding propagation angles (measured against the normal direction) are θ1 and θ2 (see Figure 1).
For example, the amplitude transmission coefficient is ts for s polarization, i.e., if the electric field vector is perpendicular to the plane of incidence.
The power reflection coefficients are obtained simply by taking the modulus squared of the corresponding amplitude coefficients. For the transmission, one must add a factor (n2 cos θ2) / (n1 cos θ1) in order to take into account the different propagation angles.
Figure 2 shows in an example case how the reflectivity of the interface depends on the angle of incidence and the polarization. The reflection coefficient vanishes for p polarization if the angle of incidence is Brewster's angle (here: ≈55.4°).
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