Polarization (optical)

See also: Polarization

The polarization is a property of the vectorial Onde S such as the Lumière. The fact that these waves are characterized by Vecteur S differentiates them other types of waves like the sound waves, and implies this phenomenon of polarization.

Theory

Explanation of the phenomenon

The simplest demonstration of polarization is that of a plane Onde, which is a good approximation of the majority of the light waves. As any electromagnetic wave which is propagated, it consists of a Electric field and of a Magnetic field both perpendiculars with the direction of propagation:

One decides, by convention, to be unaware of the magnetic field thereafter, because it can be given starting from the electric field. One thus considers only the electric field $\backslash \; vec\; E$ perpendicular to the direction of propagation .

The figure described by the electric field is then a ellipse, which can become a Cercle, or to be flattened in a line. These various forms define the state of polarization of the wave: it is said that the wave is polarized elliptically , circularly or rectilignement . The three figures below illustrate the evolution of the electric field in these three cases.

Moreover, this wave is propagated , therefore the ellipse described by the field $\backslash \; vec\; E$ is actually a Hélice, as the figures show it below (in three dimensions).

This phenomenon is explained thanks to the equation of propagation of the light wave. By breaking up the electric field into its two component orthogonal (in green and red on the preceding diagrams), one realizes that they have both a sinusoidal evolution . When the two components oscillate at the same time, a rectilinear polarization is obtained. If they present Dephasing (i.e. one is late compared to the other), then an elliptic polarization is obtained.

In the particular case where this dephasing is worth 90° and which both components have even Amplitude, polarization is circular. One then distinguishes the cases from polarization circular left and circular right-hand side according to the direction of course.

Remark : This decomposition in two orthogonal components is arbitrary. It is also possible, for example, to break up into two circular polarizations turning in contrary direction.

The incoherent light

The explanation above is valid strictly only for one wave Monochromatique, and more particularly a coherent light . In the other cases, the effects are a little different.

For example, in the light of a bulb to filament, one observes that the light is nonpolarized : it does not correspond to any of the three preceding cases. That is explained by the fact that this light is made up of wave trains, short luminous impulses having each one an amplitude, a Fréquence, and a polarization Aléatoire S. Ainsi, which we observe is a Moyenne of all these wave trains which appears nonpolarized to us.

However, the incoherent light can also present, partly, an elliptic, circular, or rectilinear polarization. To determine then which is the polarization of a light, one uses Polariseur S and blades with delay.

Physical effects on polarization

Polarization after reflection

The reflection of the light on certain materials transforms its polarization. An important example is that of the vitreous Réflexion which makes that one sees reflections on the windows. This reflection is not identical according to the polarization of the incidental light on glass. To include/understand it, one breaks up the polarization of the light into two orthogonal rectilinear polarizations between them, noted S and p . The polarization S is perpendicular to the Plan of incidence, and the polarization p is contained in this plan. The light is more or less considered according to whether it is polarized S or p , and according to the angle of incidence. That allows, for example, with the photographer, to eliminate most of the reflections on a window when he wants to photograph what there is behind. For that, it is enough to place a Polariseur in front of the camera, and to be placed at the good visual angle, called Angle of Brewster.

The reflection on metals also has an effect on polarization.

Other effects

The waves presenting a polarization can undergo various effects:

• a Polariseur by absorption makes it possible to transform an unspecified light into a light polarized rectilignement.
• the Biréfringence is a property of certain materials which separate a luminous ray in two rays from different polarizations. An important example in optics is the Lame with delay.
• the materials presenting a rotatory Pouvoir can make turn polarization. That allows, in Chimie, to differentiate the chiral Molécule S between them.

Polarization in nature, science and technology

In the life of the every day

The transparent and brilliant materials reflect the light partly. According to the angle of observation, this reflected light is more or less polarized. By looking it behind a polarizer, or polarizing sunglasses, one can see variations of the luminous intensity.

The light of the sky is also partly polarized. Therefore the photographers use polarizers in order to obscure the sky on the Photographie S, and to increase the Contraste. This effect is quite visible with laying down sun: with 90° of the sun, the light is particularly polarized. In addition, the reflection on a horizontal surface (sea, ice) polarizes the light horizontally. It is to eliminate these reflections that the fishermen and the mountaineers use polarizing sunglasses (composed of vertical filter polarizing).

In Biology

Many a animal is able to perceive the polarization of the light, which they often use for navigation, because the polarization of the light of the sky is perpendicular to the direction of the Sun. This capacity is current at the Insecte S, as the Abeille S which use it to move at the time of their communicative dance. One also observed this capacity at some Espèce S of Pieuvre S, Calmar S, cuttlefish S, and Shrimp-mante S. the skin of the cuttlefish which can change quickly, and highly coloured, is partially polarized. The shrimp-mante has also the characteristic not to reflect all polarizations in the same way.

Some Vertebrate S, like the Pigeon S, can also perceive polarization.

Geology

The polarized light is used in geology in the observation of thin blades of Roche S. It is essential tools with the identification of the mineral constituting those. It is the phenomenon of Biréfringence some crystals which allows it.

Observation in relief

The polarized light is used to observe images projected in relief, by means of two projectors with which one is provided with a polarizer directed high-left and the other high-right-hand side. The spectators carry each one a pair of glasses polarizing including both " filtres" are directed in the corresponding directions. Many films in relief were thus shown with the public, in particular in amusement parks (Quoted of Sciences, Futuroscope, etc) and in several cinemas thus equipped. See for example Stéréoscopie.

Chemistry

The chiral compounds make turn the polarization of the light the beam. This rotatory Pouvoir makes it possible to separate the enantiomer S according to the direction in which they make turn polarization.

Hertzian communications

The most current polarization of the waves in the general public applications, like terrestrial television (analogical or numerical) or by satellite, is linear, horizontal or vertical polarization. One finds these two polarities in the field of the Wifi, the Radioamateurisme and in the professional transmissions.

The various components of polarization are necessary to protect the reception from signals at identical or brought closer frequencies, because that avoids Interférence S mutual insurance companies, particularly when the signals arrive in the same direction.

The terrestrial television network uses mainly horizontal polarization (H) and thus the antenna S are horizontal. In certain areas with overloaded spectrum, like the border zones, e.g. the Alsace in France, the new emitted channels are often vertically polarized (e.g. Arte, M6 and the future TNT). One thus needs there 2 rakes for the same transmitter. There does not exist antenna TV with bipolarisation in UHF collecting, and protecting, at the same time polarizations V and H. to receive 2 different polarizations emitted by the same transmitter one uses in complement a ADF sense antenna coupler UHF. The antennas (thus their plan) being placed one vertically and the other horizontally.

Out of satellite television it is the universal Tête, ordered by the Terminal DVB-S, or the Démodulateur, which selects the plan of the polarities emitted by the antennas of the satellite, mainly rectilinear H or V. a bad selectivity (rotation of the head in the support) led to dysfunctions.

Circular polarization (intuitive: corkscrew) is used, one finds it however at the radio amateurs and in the majority of the apparatuses of transmission of living room (transmitting AV without wire) functioning in the Band 2,4 GHz (Bande S).

Radars and Teledetection

See also: Radar, weather Radar

The Radar S use polarization to extract from the extra informations to the position (obtained thanks to the Réflectivité) and to the displacement (obtained thanks to the Doppler effect) of the probed targets. Indeed, the intensities and Déphasage S between two orthogonally polarized beams ghost of the same target can reveal its form or its type.

* If the targets have a form flattened as in the image opposite, while probing with two waves of which one is of vertical polarization (V) and the other horizontal one (H), one obtains to stronger intensities ghost of that having the horizontal axis. On the other hand if the orthogonal returns are equal that indicates a round target. That is called the difference of Réflectivité ($Z\_\; \{Dr.\}$).

* the beam radar probes a more or less large volume according to the characteristics of the transmitting antenna. What cost is the addition of the waves reflected by the individual targets in volume. As the targets can change position in time the ones compared to the others, the intensity of the waves V and H remains constant only if the targets have all the same form. The report/ratio of intensity between the channels H and V returning of successive surveys is called the coefficient of correlation ($\backslash \; rho\_\; \{hv\}$) and thus gives an idea of the homogeneity or not targets in probed volume.

* the phase of the wave changes when it crosses a medium of different density. By comparing the rate of phase shift of the wave of return with the distance, the specific differential phase or $K\_\; \{dp\}$, one can evaluate the quantity of crossed matter.

* One can also compare dephasing between the returns H and V ( differential of phase or $\backslash \; phi\_\; \{dp\}$).

An example of use is that by the weather radars. Probed volumes contain a multitude of targets such drop lettes, flake S or let us hail. They all can be of the same type or be a mixture of forms and types. By using various information of the polarized waves, one thus obtains extra informations which can give directly us the type of Hydrométéore S when one analyzes them in conjunction with the reflectivity. This even helps to eliminate the undesirable Artéfact S like the birds or the echoes returning from the obstacles like the mountains.

Other types of radar use polarization at various ends. For example, the Radar with synthesis of opening is used to chart areas by plane or satellite and uses polarization to distinguish the types of ground, vegetation, etc

Astronomical observations

The purpose of the experiment PILE is measurement of the polarization of the emission of galactic dust in remote IR. During the next decade, the very precise measurement of the polarization of the cosmological bottom (CMB) will be one of the major stakes for cosmology and fundamental physics. The main aim will be the measurement of the modes “B” of the CMB, which will inform us at the same time about the bath of paramount gravitational waves generated for the period of inflation of the universe which took place in the very first moments of the expansion and on the structuring of the matter after the big-bang (effect of microlentille on the bottom). The Department Cold Universe of the Center of Space studies of the Radiations takes this route resolutely, and takes an active part in several projects of preparation, in particular the experiment balloon PILE.

Official site of the PILE project

Screen with liquid crystals

See also: Screen with liquid crystals

The screens with liquid crystals became current since the Années 1980. Initially used for monochromic posting on the dials and watches, they have been found now in televisions, the numeric cameras and any other posting thanks to their compactness, and, for a few years, their polychromy. The screen uses the polarization of the light thanks to polarizing filters and to the Biréfringence of certain liquid crystals in nematic phase which one can vary the orientation according to the electric field.

Screen LCD ( Liquid Crystal Display ) consists of two polarizers whose directions of polarization form an angle of 90°, on each side of a formed sandwich of two glass plates enclosing of the Liquid crystals. The two inner faces of the glass plates comprise a matrix of transparent electrodes, one (black and white) or three (color) by Pixel. The thickness of the device and the nature of liquid crystals are selected so as to obtain the desired rotation of the plan of polarization in the absence of electric tension.

The application of a Potential difference more or less high between the two electrodes of a pixel involves a change of orientation of the molecules, a variation of the plan of polarization and thus a variation of the transparency of the whole of the device. This variation of transparency is exploited by a retro-lighting, reflection of the incidental light or by projection.

Small monochromic bill-posters LCD often use front electrodes in the form of segments of character in order to simplify electronics (direct direct order in all or nothing) while obtaining a very good legibility (not sweeping). More complex postings, as for the Television S, use electrodes by pixels accessible by whole line or column. Their extinction or spark control must be done by a regular sweeping of the lines/columns of points.

See too

Related articles

• electromagnetic Wave

External bonds

• a video explanatory on the various states of polarization of the light

• a video explanatory on different the way of polarizing the natural light

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