= Rotational Doppler effect
A <frequency> change associated with rotation and exchange of <angular momentum>. If an optical element rotates at angular velocity $\Omega$ about a chosen axis and gains <angular momentum> $\Delta L$ from a <photon>, its mechanical <energy> gain is $\Omega\Delta L$ in the prescribed slow-rotation model. <Energy> conservation then gives <photon> <frequency> change $\Delta\omega=-\Omega\Delta L/\hbar$. The <rotating half-wave plate frequency shift> is a spin-polarization example.
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