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.