where θ is the incident angle between the propagation vector of the incident field and the normal to the surface and <math>\eta_0 = 120\pi \; \Omega</math> is the intrinsic impedance of the free space.
It From the surface impedance boundary condition, it can easily be shown that
:<math> \mathbf{M(r)} = -Z_s \mathbf{\hat{n}\times} \mathbf{J(r)} </math>
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In the case of an impedance-matched surface (Z<sub>s</sub> = η<sub>0</sub>), one can write: