Circular polarization

At any instant of time, the electric field vector of the wave indicates a point on a helix oriented along the direction of propagation.

All three terms were coined by Augustin-Jean Fresnel, in a memoir read to the French Academy of Sciences on 9 December 1822.

[3] The phenomenon of polarization arises as a consequence of the fact that light behaves as a two-dimensional transverse wave.

Circular polarization occurs when the two orthogonal electric field component vectors are of equal magnitude and are out of phase by exactly 90°, or one-quarter wavelength.

Circular polarization is often encountered in the field of optics and, in this section, the electromagnetic wave will be simply referred to as light.

The result of this alignment are select vectors, corresponding to the helix, which exactly match the maxima of the vertical and horizontal components.

To appreciate how this quadrature phase shift corresponds to an electric field that rotates while maintaining a constant magnitude, imagine a dot traveling clockwise in a circle.

Consider how the vertical and horizontal displacements of the dot, relative to the center of the circle, vary sinusoidally in time and are out of phase by one quarter of a cycle.

Now referring again to the illustration, imagine the center of the circle just described, traveling along the axis from the front to the back.

The next pair of illustrations is that of left-handed, counterclockwise circularly polarized light when viewed by the receiver.

Aside from the reversal of handedness, the ellipticity of polarization is also preserved (except in cases of reflection by a birefringent surface).

Only in the special case of normal incidence, where there is no distinction between p and s, are the Fresnel coefficients for the two components identical, leading to the above property.

Circular polarization may be referred to as right-handed or left-handed, and clockwise or anti-clockwise, depending on the direction in which the electric field vector rotates.

Using this convention, left- or right-handedness is determined by pointing one's left or right thumb toward the source, against the direction of propagation, and then matching the curling of one's fingers to the temporal rotation of the field.

Note that, in the context of the nature of all screws and helices, it does not matter in which direction you point your thumb when determining its handedness.

As a general rule, the engineering, quantum physics, and radio astronomy communities use the first convention, in which the wave is observed from the point of view of the source.

[5][7][8] In many physics textbooks dealing with optics, the second convention is used, in which the light is observed from the point of view of the receiver.

FM broadcast radio stations sometimes employ circular polarization to improve signal penetration into buildings and vehicles.

It is one example of what the International Telecommunication Union refers to as "mixed polarization", i.e. radio emissions that include both horizontally- and vertically-polarized components.

Circular dichroism is the basis of a form of spectroscopy that can be used to determine the optical isomerism and secondary structure of molecules.

The maximum absolute value of gem, corresponding to purely left- or right-handed circular polarization, is therefore 2.

Meanwhile, the smallest absolute value that gem can achieve, corresponding to linearly polarized or unpolarized light, is zero.

In the case of circular polarization, the electric field vector of constant magnitude rotates in the x-y plane.

If basis vectors are defined such that: and: then the polarization state can be written in the "R-L basis" as: where: and: A number of different types of antenna elements can be used to produce circularly polarized (or nearly so) radiation; following Balanis,[16] one can use dipole elements: "... two crossed dipoles provide the two orthogonal field components....

Also, if the two dipoles were fed with a 90° degree time-phase difference (phase quadrature), the polarization along zenith would be circular.... One way to obtain the 90° time-phase difference between the two orthogonal field components, radiated respectively by the two dipoles, is by feeding one of the two dipoles with a transmission line which is 1/4 wavelength longer or shorter than that of the other," p.80;or helical elements: "To achieve circular polarization [in axial or end-fire mode] ... the circumference C of the helix must be ... with C/wavelength = 1 near optimum, and the spacing about S = wavelength/4," p.571;or patch elements: "... circular and elliptical polarizations can be obtained using various feed arrangements or slight modifications made to the elements.... Circular polarization can be obtained if two orthogonal modes are excited with a 90° time-phase difference between them.

The quadrature phase difference is obtained by feeding the element with a 90° power divider," p.859.In the quantum mechanical view, light is composed of photons.

In 1911, Albert Abraham Michelson discovered that light reflected from the golden scarab beetle Chrysina resplendens is preferentially left-polarized.

Since then, circular polarization has been measured in several other scarab beetles such as Chrysina gloriosa,[18] as well as some crustaceans such as the mantis shrimp.

If this light is then totally internally reflected back down, its vertical component undergoes a phase shift.

To an underwater observer looking up, the faint light outside Snell's window therefore is (partially) circularly polarized.