Editing Circular polarization (section)

== Characteristics ==
{{multiple image
 | direction = vertical
 | footer  = Right-handed/counterclockwise circularly polarized light displayed with and without the use of components. This would be considered clockwise circularly polarized if defined from the point of view of the source rather than the receiver. Handedness is independent of the perspective of the source or receiver.
 | footer_align = left
 | width   = 440
 | image1  = Circular.Polarization.Circularly.Polarized.Light_Without.Components_Right.Handed.svg
 | image2  = Circular.Polarization.Circularly.Polarized.Light_With.Components_Right.Handed.svg
 | image3 =  Circular polarization cross section.gif
}}

In a circularly polarized electromagnetic wave, the individual electric field vectors, as well as their combined vector, have a constant [[Magnitude (vector)|magnitude]], and with changing phase angle. Given that this is a [[plane wave]], each vector represents the magnitude and direction of the electric field for an entire plane that is perpendicular to the optical axis. Specifically, given that this is a [[Plane wave#Polarized electromagnetic plane waves|circularly polarized plane wave]], these vectors indicate that the electric field, from plane to plane, has a constant strength while its direction steadily rotates. Refer to [[Sinusoidal_plane_wave#Polarized electromagnetic plane waves|these two images]] in the plane wave article to better appreciate this dynamic. This light is considered to be right-hand, clockwise circularly polarized if viewed by the receiver. Since this is an [[Electromagnetic radiation|electromagnetic wave]], each [[electric field]] vector has a corresponding, but not illustrated, [[magnetic field]] vector that is at a [[right angle]] to the electric field vector and [[Proportionality (mathematics)|proportional]] in magnitude to it. As a result, the magnetic field vectors would trace out a second helix if displayed.

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 nature of circular polarization and its relationship to other polarizations is often understood by thinking of the electric field as being divided into two [[Euclidean vector|components]] that are perpendicular to each other. The vertical component and its corresponding plane are illustrated in blue, while the horizontal component and its corresponding plane are illustrated in green. Notice that the rightward (relative to the direction of travel) horizontal component leads the vertical component by one quarter of a [[wavelength]], a 90° phase difference. It is this [[In-phase and quadrature components|quadrature phase]] relationship that creates the [[helix]] and causes the points of maximum magnitude of the vertical component to correspond with the points of zero magnitude of the horizontal component, and vice versa. 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 (waves)|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 [[Displacement (vector)|displacements]] of the dot, relative to the center of the circle, vary [[Sine wave|sinusoidally]] in time and are out of phase by one quarter of a cycle. The displacements are said to be out of phase by one quarter of a cycle because the horizontal maximum displacement (toward the left) is reached one quarter of a cycle before the vertical maximum displacement is reached. 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 circling dot will trace out a helix with the displacement toward our viewing left, leading the vertical displacement. Just as the horizontal and vertical displacements of the rotating dot are out of phase by one quarter of a cycle in time, the magnitude of the horizontal and vertical components of the electric field are out of phase by one quarter of a wavelength.

{{multiple image
| align = right
| direction = vertical
| footer = Left-handed/counterclockwise circularly polarized light displayed with and without the use of components. This would be considered right-handed/clockwise circularly polarized if defined from the point of view of the source rather than the receiver.
| footer_align = left
| width = 440
| image1 = Circular.Polarization.Circularly.Polarized.Light_Without.Components_Left.Handed.svg
| caption1 = 
| image2 = Circular.Polarization.Circularly.Polarized.Light_With.Components_Left.Handed.svg
| caption2 = 
}}
The next pair of illustrations is that of left-handed, counterclockwise circularly polarized light when viewed by the receiver. Because it is left-handed, the rightward (relative to the direction of travel) horizontal component is now ''lagging'' the vertical component by one quarter of a wavelength, rather than leading it.

===Reversal of handedness===

====Waveplate====
To convert circularly polarized light to the other handedness, one can use a half-[[waveplate]]. A half-waveplate shifts a given linear component of light one half of a wavelength relative to its orthogonal linear component.

====Reflection====
The handedness of polarized light is reversed reflected off a surface at normal incidence. Upon such reflection, the rotation of the [[plane of polarization]] of the reflected light is identical to that of the incident field. However, with propagation now in the ''opposite'' direction, the same rotation direction that would be described as "right-handed" for the incident beam, is "left-handed" for propagation in the reverse direction, and vice versa. Aside from the reversal of handedness, the ellipticity of polarization is also preserved (except in cases of reflection by a [[Birefringence|birefringent]] surface).

Note that this principle only holds strictly for light reflected at normal incidence. For instance, right circularly polarized light reflected from a dielectric surface at grazing incidence (an angle beyond the [[Brewster's angle|Brewster angle]]) will still emerge as right-handed, but elliptically polarized. Light reflected by a metal at non-normal incidence will generally have its ellipticity changed as well. Such situations may be solved by decomposing the incident circular (or other) polarization into components of linear polarization parallel and perpendicular to the [[plane of incidence]], commonly denoted ''p'' and ''s'' respectively. The reflected components in the ''p'' and ''s'' linear polarizations are found by applying the [[Fresnel equations|Fresnel coefficients]] of reflection, which are generally different for those two linear polarizations. 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.

[[File:Reversal of handedness of circularly polarized light reflected by mirror 2s.gif|thumbnail|A 3-slide series of pictures taken with and without a pair of MasterImage 3D circularly polarized movie glasses of some dead European rose chafers (Cetonia aurata) whose shiny green color comes from left-polarized light. Note that, without glasses, both the beetles and their images have shiny color. The right-polarizer removes the color of the beetles but leaves the color of the images. The left-polarizer does the opposite, showing reversal of handedness of the reflected light.]]

===Conversion to linear polarization===
Circularly polarized light can be converted into linearly polarized light by passing it through a quarter-[[waveplate]]. Passing linearly polarized light through a quarter-waveplate with its axes at 45° to its polarization axis will convert it to circular polarization. In fact, this is the most common way of producing circular polarization in practice. Note that passing linearly polarized light through a quarter-waveplate at an angle ''other'' than 45° will generally produce elliptical polarization.