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I came across a problem whose answer I can't figure out:

Under what circumstances do two beams of light interfere and create a polarized beam of light?

Shouldn't this always happen, considering 2 beams need a constant phase difference in order for interference to occur? Sorry if this question has been answered before; I didn't find any discussion immediately relevant.

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    $\begingroup$ Can you add some context? It's possible for two circularly polarized beams of opposite chirality to interfere and produce a linearly polarized beam. Basically, any way you can decompose a linearly polarized beam into two beams of different polarization and phase is reversible: those two beams, combined, produce the linearly polarized beam. $\endgroup$ – S. McGrew Jun 24 '18 at 15:25
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The short answer is: one of the polarization components need to interfere destructively.

One can always express any state of polarization as a superposition of two mutually orthogonal states of polarization, such a horizontal and vertical linear states of polarization or left- and right circular polarization. In this way the polarization is represented in terms of two components.

If you prepare the states of polarization of two beams so that one of these components are in phase ebtween the two beams, while the other is out of phase, then, upon combining the two beams, the components that are in phase will interfere constructively and the component that is out of phase will interfere destructively. The result would be that only the component of the state of polarization that interfered constructively would survive. In this way interference changed the state of polarization.

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