= Power transmission coefficient
{title2=$\mathcal T=P_{\rm transmitted}/P_{\rm incident}$}
The ratio of transmitted to incident normal time-averaged <power> for a specified wave and interface. Converting a field-amplitude transmission coefficient into power requires the appropriate impedances or flux <normalization>. For a displacement wave on a string with impedance $Z$, $\mathcal T=(Z_2/Z_1)|t|^2$; using an electric-field amplitude in electromagnetic waves instead gives the corresponding electromagnetic flux ratio. Amplitude squared alone is sufficient only when the relevant incident and transmitted <normalizations> agree.
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