= Rotatory power
{title2=$\rho=d\theta/d\ell=\pi(n_+-n_-)/\lambda_0$}
The <rotatory power> is the rotation of transmitted <linear polarization> per unit path length, in $\mathrm{rad\,m^{-1}}$. With equally transmitted circular eigenmodes $(\mathbf e_1\mp i\mathbf e_2)/\sqrt2$ and time convention $e^{-i\omega t}$, their accumulated <phase difference> gives $\theta=(\phi_+-\phi_-)/2$. For uniform <circular birefringence>, $\rho=\pi(n_+-n_-)/\lambda_0$. Generic linear <birefringence> need not admit one rotation angle common to all incident polarization directions. In a weak longitudinal <Faraday effect>, $\rho=VB_\parallel$, with signed <Verdet constant> $V$.
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