= Reflection-amplitude sign convention
With one fixed tangential electric-field direction, normal-incidence reflection from impedance $Z_1$ to $Z_2$ gives $r_E=(Z_2-Z_1)/(Z_2+Z_1)$. Using one fixed tangential magnetic-field direction reverses the reflected wave's sign, giving $r_H=-r_E$. Reversing the chosen reflected basis can produce the same change. The <reflectance> $|r|^2$ is invariant, but coherent amplitudes require a consistent convention throughout. Two internal reflections at identical opposite interfaces give the same round-trip product in either convention.
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