Polarization (waves)

The polarization of a transverse wave describes the direction of oscillation. These changes direction quickly and randomly, one speaks of an unpolarized wave. The degree of polarization indicates the parent share. With vibrating waves in the propagation direction, the longitudinal wave, there is no polarization.

Components that polarize unpolarized light or split polarized light depending on the type and direction of polarization or suppress hot polarizer.

Types of polarization

• Linear polarization: The direction of oscillation is constant. The displacement from the equilibrium position (in the case of the mechanical wave displacement perpendicular to the propagation direction) periodically changes its magnitude and its sign. Direction with respect to a certain level can be specified as an angle (for conventional seismic waves ), or in parallel as a percentage of the two components or vertically. For electromagnetic waves, see the following chapter.
• Circular polarization ( in the 19th century called circular polarization ): The amount of deflection is (apart from modulation ) is constant, its direction changes within the wave vector perpendicular to the plane standing ( the xy plane in the picture) with a constant angular velocity. For the rotation See helicity. Abbreviations: RHCP and LHCP for right - or left-handed polarization ( eng. Right Hand Circular Polarization and Left Hand Circular Polarization ).
• Elliptical polarization is a mixed form. The deflection here describes an ellipse.

Mathematical Description

• Composition of a linear, circular or elliptically polarized wave ( black) of linearly polarized components ( red and blue)

Any polarization can be represented as the superposition of two base polarizations. In circularly polarized light, the spins of all photons show in the same direction, depending on the helicity either in or against the direction of propagation. Nevertheless, a single photon can be linearly polarized by two oppositely circularly polarized states are superimposed.

Notwithstanding but it is possible and common to present circularly polarized wave as a superposition of linearly polarized components. Common to both types is the specification of amplitude ratio and relative phase position:

• At zero phase difference ( or a phase difference corresponding to a multiple of π ), and of different or same amplitude, the output linear polarization and the direction depends on the amplitude ratio.
• At a phase difference of π / 2 and equal amplitudes, the output polarization is circular.
• In any other case, the output polarization is elliptical.

• With the same intensities and variable phase difference, the output polarization is linear, and the direction is dependent on the phase position of the base polarizations.
• In any other case, the output polarization is elliptical.

Is the absolute intensity and the degree of polarization of interest, so a total of four pieces of information are necessary as four-dimensional Stokes vector of real-valued or complex-valued two-dimensional as a Jones vector. Quasimonochromatisches light may alternatively be described by the consistency matrix. The description of the operation of a polarization -changing optical element is then carried out by multiplying by a corresponding Mueller matrix or a Jones matrix.

Polarization of electromagnetic waves

Unpolarized and polarized light in nature

Since no individual particles or waves are considered in view of electromagnetic radiation in general, it is the case of electromagnetic radiation, usually by the superposition of a plurality of single waves with different location of the vibration level and relative phase. Most of the light occurring in nature is initially unpolarized as thermal radiation, that is, the individual waves are statistically distributed in their properties. By reflection or scattering thereof occurs partially polarized light, wherein a part of the single waves same properties in terms of their polarization, and appears as a polarization pattern.

Weird reflection at interfaces, eg on a water surface, light separates partly after its polarization direction. The polarized in the plane of reflection portion penetrates rather, the vertical component is likely to be reflected. For the quantitative function of the incidence angle see Fresnel formulas.

The blue light of the sky is scattered by molecules and statistical density fluctuations in air sunlight. The air is electrically polarized by the incident wave in random directions perpendicular to the direction of incidence. Scattered light in directions that vibration level ( scattering angle 90 °) oscillates in this very level and is completely polarized. For the function of the scattering angle see Rayleigh scattering. The light from the sky is not completely polarized by multiple scattering and scattering by particles.

Perception of polarized light

Many insects can distinguish linearly polarized light according to its polarization direction and use this effect to orient himself. For the honey bee it has been researched by Karl von Frisch. Also, mantis shrimp, the water bug Notonecta glauca and desert ants are capable of doing, as well as people, but with very low contrast ( Haidinger - Tufts phenomenon).

Names of the sub-groups of the polarization modes

Electromagnetic radiation ( light, radio waves, etc.), a transverse wave with in each case at right angles between the wave vector pointing in the propagation direction, and the vectors of the electric and magnetic field, respectively. It is arbitrary whether the vibrating direction of the electric or the magnetic field is selected as the polarization direction. From the time has been explained as light as a mechanical vibration of the hypothetical ether, comes a definition for the names of the two polarization directions, which turned out later than the oscillation direction of the magnetic field vector. Since most of interactions of electromagnetic radiation with matter, however, are electrical in nature, the polarization direction is now mainly based on the electric field vector.

If the wave is refracted, reflected or scattered, the reference plane is parallel and perpendicular to the names that plane in which lie the incoming and the outgoing wave. In the case of radio waves, the earth's surface, the " reference level" is hot and the components horizontally and vertically in the rule.

Next parallel and perpendicular polarized waves other names can be used, inter alia in the description of reflection. Thus one speaks of TM- polarized light when the vibration plane of the magnetic field perpendicular to the plane spanned by the incident vector and the surface normal plane ( " plane of incidence " ) lies ( TM = transverse magnetic, we here also speaks of parallel -, p- or π - polarized light ), and TE - polarized light when the electric field is perpendicular to the plane of incidence (TE transverse electric, is in this case also referred to as vertical -, s-or σ - polarized light). In the direction of the Brewster angle TM -polarized light is amplified broken rather than reflected in the media, that is, even for unpolarized incident light the precipitating at Brewster angle light is always TE -polarized. Both terms are defined only in connection with the reflecting surface.

With circularly or elliptically polarized light is different, the propagation direction with respect to the rotational direction of the E- or H- vector in the reference. This is known as left - or right-handed polarized light.

In laser spectroscopy, the circular polarization is split instead of the right and left ( " Sigma -Plus ") and ( " Sigma -minus "), which causes a change in the magnetic quantum number of 1 or -1 in atomic transitions between energy levels. Linearly polarized light (in the atomic transition ) is called polarized light. Advantage of this specification is the independence of the coordinate system (right and left); Instead, it refers to the chosen quantization axis of the atom.