Polarization of Electromagnetic Waves  

In General > see light; optics / electromagnetism.
* Remark: Unpolarized light must be polychromatic!
* Malus' law: The intensity of polarized light passing through a linear polarizer decreases by a factor cos2α; > s.a. hidden variables.
* Of sky light: The scattering that causes the blue sky (mostly single scattering) also polarizes it; To first approximation there are two 0-polarization points, the Sun and the anti-Sun, but double scattering causes them to split vertically into two closely spaced points (Brewster and Babinet points, each of index 1/2).
* Demonstration: Black light sandwich – a transparency foil between two polarizers, allows to see two index-1/2 singularities in direction space.
@ General references: Pye 00 [I]; Gamel & James PRA(12)-a1303 [degree of polarization, and degree of purity for two-level quantum systems]; Trippe JKAS(14)-a1401 [rev, and astronomy].
@ Stokes polarization parameters: Schaefer et al AJP(07)feb; > s.a. gravitational-wave background.
@ Polarization of skylight: Smith AJP(07)jan [model]; Ropars et al CP(14) [and the sunstone as an ancient Viking navigational tool].

Faraday Rotation > s.a. cmb polarization.
* Idea: The rotation of the plane of polarization of light going through a magnetic field (or other medium); > s.a. electromagnetism in curved spacetime.
* Gravitational: The rotation of the plane of polarization of polarized electromagnetic radiation traveling through a gravitational field; In the case of a gravitational wave, the effect vanishes for localized (astrophysically generated) waves and is non-zero but negligible for cosmological ones.
@ General references: Ganguly et al PRD(99) [degenerate relativistic]; Zavattini PRL(06) + pw(06)mar [observation, and axions]; Cornean & Nenciu JFA(09)-a0807 [thermodynamic limit]; Melrose ApJ(10)-a1010 [effect of magnetic-field reversals]; Oppermann et al A&A(15)-a1404 [extragalactic].
@ Gravitational: Perlick & Hasse CQG(93); Sereno PRD(04); Halilsoy & Gurtug PRD(07), Faraoni NA(07)-a0709 [due to gravitational waves].

Other Effects > s.a. Coherence; earth [auroras].
* Birefringence: A polarization-dependent doubling in photon propagation speed, or anisotropic index of refraction; It happens in material media such as calcite, and in vacuum either as a quantum field theory effect in the presence of strong electromagnetic fields, or in modified gravity theories such as bi-metric models and Poincaré gauge theory; A similar effect can occur for gravitational waves; > s.a. axions; finsler geometry; modified electromagnetism; non-commutative gauge theories; QCD phenomenology.
* Dichroism: Anisotropic absorption of components of an electromagnetic wave; For example, by a grid of parallel conductors.
* Optical activity: The rotation of the electric field of a plane wave traversing some materials (e.g., quartz).
@ Optical activity: Barbieri & Guadagnini NPB(04) [gravitational]; Agulló et al a1705-GRF [in a curved spacetime geometry, for quantum radiation].
@ Birefringence, material medium: Ghosh & Fischer PRL(06) + pw(06)nov [in a chiral liquid].
@ Birefringence, vacuum: Chen et al MPLA(07)he/06 [and vacuum dichroism, experimental searches]; Hattori & Itakura AP(13) [in strong magnetic fields]; Karbstein et al PRD(15)-a1507 [in strong inhomogeneous electromagnetic fields]; Mignani et al MNRAS(16)-a1610 + news sn(16)dec [in X-rays from neutron stars]; Karbstein a1611-ln [pedagogical].
@ Birefringence, cosmological: Exirifard PLB(11)-a1010 [theory]; di Serego Alighieri a1011-proc [methods, rev]; Yadav et al PRD(12)-a1207 [with cmb], Gruppuso et al JCAP(16)-a1509 [constraints from Planck and Bicep2/Keck data]; Duval & Schücker a1610 [in Robertson-Walker cosmologies]; Whittaker et al a1702/MNRAS [using resolved radio sources].
@ Experiments and examples: Ouseph et al AJP(01)nov [polarization by reflection]; Holleczek et al OE(11)-a1012 [cylindrically polarized states of light].
@ Related topics: Berry & Dennis PRS(01) [polarization singularities in vector waves]; news sn(10)mar ["superchiral" light]; Shukla & Prakash MPLB(16)-a1610 [polarization squeezing].
> In gravitation and cosmology: see kaluza-klein phenomenology; non-commutative physics; Non-Symmetric Gravity; gravitational phenomenology; observational cosmology; photon phenomenology in quantum gravity; tests of general relativity with light; tests of the equivalence principle.

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