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When circularly polarized light is passed through a quarter-wave plate, the plate experiences a torque. I understand this is true because angular momentum must be conserved, but I don't understand what is happening in the plate. From a classical perspective, how is the light interacting with the electrons in the plate in order to create the torque? I prefer a physical description over a mathematical one. Thanks.

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The global polarization state of the light is the result of the spins of individual photons. Spin is an angular momentum ($\pm \hbar$). Passing through a medium, the polarization changes if the spin of individual photons -and the associated angular momentum- is affected. This can happen by birefringence (visualize it as a interference-dependent "twist" of the photon from $\hbar$ to $-\hbar$ if you like) or even absorption ($\hbar$ to just $0$). The change in angular momentum is the torque applied or transferred to the plate.

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  • $\begingroup$ Thanks. Can you provide a description of this process in terms of waves and electric fields? I understand the concept of massless particles exchanging momentum with the plate, but it's a little abstract for me. I'm trying to understand where exactly there is a force and moment-arm that creates the torque. $\endgroup$ – James Jan 27 '18 at 16:11

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