= Waveplate retardance detuning
{title2=$\epsilon\simeq(d/c)(\Delta n_0+\omega_0\Delta n'_0)\Delta\omega$}
A zero-order <quarter-wave plate> satisfies $\omega_0d\Delta n(\omega_0)/c=\pi/2$. Its <retardance> error at nearby <angular frequency> is $\epsilon=\omega d\Delta n(\omega)/c-\pi/2$. Taylor expansion gives the title expression, including material dispersion. Neglecting that dispersion yields $\epsilon=(\pi/2)\Delta\omega/\omega_0$. A <half-wave plate> made from twice the same material thickness has exactly twice this <phase> error. Common propagation <phase> can be removed from its <Jones matrix> without changing <polarization>.
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