= Absorbing Fabry–Pérot mirror phase and transmission
{title2=$\delta=4\pi\nu nd/c+2\arg r$}
For identical mirrors with power reflectivity $\mathcal R$, transmission $\mathcal T$ and absorption $1-\mathcal R-\mathcal T$, summing round trips at <normal incidence> gives
$$
\frac{I_t}{I_0}=\frac{\mathcal T^2}{(1-\mathcal R)^2+4\mathcal R\sin^2(\delta/2)}.
$$
Here $\delta$ includes propagation and both internal reflection phases. <Optical absorption> reduces the resonant peak below unity, while a nonzero reflection phase shifts the resonance condition $\delta=2\pi m$. Frequency-dependent mirror phase and <refractive index> can also change the <free spectral range>.
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