:<math> R_{\|} = \frac{\cos\theta - \eta_s } {\cos\theta + \eta_s } </math>
where η<sub>s</sub> = Z<sub>s</sub>/η<sub>s</sub>, θ is the incident angle between the propagation vector of the incident field and the normal to the surface , <math>\eta_s = Z_s/\eta_0</math> and <math>\eta_0 = 120\pi \; \Omega</math> is the intrinsic impedance of the free space.
From the surface impedance boundary condition, it can easily be shown that