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Inelectrodynamics,elliptical polarization is thepolarization ofelectromagnetic radiation such that the tip of theelectric fieldvector describes anellipse in any fixed plane intersecting, andnormal to, the direction of propagation. An elliptically polarized wave may be resolved into twolinearly polarized waves inphase quadrature, with their polarization planes at right angles to each other. Since the electric field can rotate clockwise or counterclockwise as it propagates, elliptically polarized waves exhibitchirality.

Circular polarization andlinear polarization can be considered to be special cases ofelliptical polarization. This terminology was introduced byAugustin-Jean Fresnel in 1822,^[1] before the electromagnetic nature of light waves was known.

Theclassicalsinusoidal plane wave solution of theelectromagnetic wave equation for theelectric andmagnetic fields is (Gaussian units)

${\displaystyle \mathbf{E}(\mathbf{r},t)=\left|\mathbf{E}\right|\mathrm{R}\mathrm{e}\left\{|\psi ⟩\exp \left[i\left(kz-\omega t\right)\right]\right\}}$

${\displaystyle \mathbf{B}(\mathbf{r},t)=\hat{\mathbf{z}}\times \mathbf{E}(\mathbf{r},t),}$

where${\displaystyle k}$ is thewavenumber,$\omega =ck$ is theangular frequency of the wave propagating in the +z direction, and${\displaystyle c}$ is thespeed of light.

Here${\displaystyle |\mathbf{E}|}$ is theamplitude of the field and

${\displaystyle |\psi ⟩\stackrel{\mathrm{d}\mathrm{e}\mathrm{f}}{=}\left(\begin{array}{c}{\psi }_x\\ {\psi }_y\end{array}\right)=\left(\begin{array}{c}\cos \theta \exp \left(i{\alpha }_x\right)\\ \sin \theta \exp \left(i{\alpha }_y\right)\end{array}\right)}$

is the normalizedJones vector. This is the most complete representation of polarized electromagnetic radiation and corresponds in general to elliptical polarization.

At a fixed point in space (or for fixed z), the electric vector${\displaystyle \mathbf{E}}$ traces out an ellipse in the x-y plane. The semi-major and semi-minor axes of the ellipse have lengths A and B, respectively, that are given by

${\displaystyle A=|\mathbf{E}|\sqrt{\frac{1+\sqrt{1-{\sin }^2(2\theta ){\sin }^2\beta }}{2}}}$

and

${\displaystyle B=|\mathbf{E}|\sqrt{\frac{1-\sqrt{1-{\sin }^2(2\theta ){\sin }^2\beta }}{2}}}$,

where${\displaystyle \beta ={\alpha }_y-{\alpha }_x}$ with the phases${\displaystyle {\alpha }_x}$ and${\displaystyle {\alpha }_y}$.The orientation of the ellipse is given by the angle${\displaystyle \varphi }$ the semi-major axis makes with the x-axis. This angle can be calculated from

${\displaystyle \tan 2\varphi =\tan 2\theta \cos \beta }$.

If${\displaystyle \beta =0}$, the wave islinearly polarized. The ellipse collapses to a straight line${\displaystyle (A=|\mathbf{E}|,B=0}$) oriented at an angle${\displaystyle \varphi =\theta }$. This is the case of superposition of two simple harmonic motions (in phase), one in the x direction with an amplitude${\displaystyle |\mathbf{E}|\cos \theta }$, and the other in the y direction with an amplitude${\displaystyle |\mathbf{E}|\sin \theta }$. When${\displaystyle \beta }$ increases from zero, i.e., assumes positive values, the line evolves into an ellipse that is being traced out in the counterclockwise direction (looking in the direction of the propagating wave); this then corresponds toleft-handed elliptical polarization; the semi-major axis is now oriented at an angle${\displaystyle \varphi \ne \theta }$. Similarly, if${\displaystyle \beta }$ becomes negative from zero, the line evolves into an ellipse that is being traced out in the clockwise direction; this corresponds toright-handed elliptical polarization.

If${\displaystyle \beta =\pm \pi /2}$ and${\displaystyle \theta =\pi /4}$,${\displaystyle A=B=|\mathbf{E}|/\sqrt{2}}$, i.e., the wave iscircularly polarized. When${\displaystyle \beta =\pi /2}$, the wave is left-circularly polarized, and when${\displaystyle \beta =-\pi /2}$, the wave is right-circularly polarized.

Any fixed polarization can be described in terms of the shape and orientation of the polarization ellipse, which is defined by two parameters: axial ratio AR and tilt angle${\displaystyle \tau }$. The axial ratio is the ratio of the lengths of the major and minor axes of the ellipse, and is always greater than or equal to one.

Alternatively, polarization can be represented as a point on the surface of thePoincaré sphere, with${\displaystyle 2\times \tau }$ as thelongitude and${\displaystyle 2\times ϵ}$ as thelatitude, where${\displaystyle ϵ=\mathrm{arccot}(\pm AR)}$. The sign used in the argument of the${\displaystyle \mathrm{arccot}}$ depends on the handedness of the polarization. Positive indicates left hand polarization, while negative indicates right hand polarization, as defined by IEEE.

For the special case ofcircular polarization, the axial ratio equals 1 (or 0 dB) and the tilt angle is undefined. For the special case oflinear polarization, the axial ratio is infinite.

The reflected light from some beetles (e.g.Cetonia aurata) is elliptical polarized.^[2]

• Ellipsometry
• Fresnel rhomb
• Photon polarization
• Sinusoidal plane-wave solutions of the electromagnetic wave equation

•  This article incorporatespublic domain material fromFederal Standard 1037C.General Services Administration. Archived fromthe original on 2022-01-22. (in support ofMIL-STD-188).

• Henri Poincaré (1889)Théorie Mathématique de la Lumière, volume 1 andVolume 2 (1892) viaInternet Archive.
• H. Poincaré (1901)Électricité et Optique : La Lumière et les Théories Électrodynamiques, via Internet Archive

• Animation of Elliptical Polarization (on YouTube)
• Comparison of Elliptical Polarization with Linear and Circular Polarizations (YouTube Animation)

• Polarization (waves)

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