What is monochromatic unpolarized light
Examples of transverse waves are electromagnetic waves, which also include light, and sound waves in solids, whereby it should be noted that there are also longitudinal sound waves in solids in addition to transverse waves.
Polarization of electromagnetic waves
To describe the polarization of electromagnetic waves, one usually refers to the electric field and ignores the magnetic field, which is perpendicular to the electric field. In circularly polarized light, the spins of all photons point in the same direction. Nevertheless, a single photon can also be linearly polarized by superimposing two oppositely circularly polarized states.
Creation of a circular polarization
Any polarization can be represented as a superposition of two basic polarizations. Most commonly used as a base:
- Two linearly polarized waves whose polarization directions are perpendicular to each other. This results in:
- Any directional linear polarizations with vanishing phase difference and variable intensities.
- circular polarization with a phase difference of π / 2 and the same intensities.
- elliptical polarization in any other case.
- A right and a left circular wave. This results in:
- Any directional linear polarizations with the same intensities and variable phase difference.
- circular polarization when one of the base amplitudes disappears.
- elliptical polarization in any other case.
Unpolarized light cannot be generated by superimposing coherent polarized waves.
Light generated by incandescent emission, such as light from the sun or from incandescent lamps, is unpolarized. It can be linearly polarized by sending it through a linear polarizer. Monochromatic linearly polarized light can be converted into circularly polarized light in a λ / 4 plate (→ phase shifter).
The light from a laser is usually polarized. The polarization can, however, be unstable, so that a (partial) polarizer (for example a Brewster window in the case of a helium-neon laser) is necessary within the resonator in order to ensure a stable and well-defined direction of polarization.
Generation of polarized light
If the molecular axes are parallel with two plastic foils connected in this way, the polarized light can penetrate through the second foil. But if they are normal (perpendicular) to one another, the polarized light coming from the first film is extinguished by the second film.
If a train of light waves that oscillates obliquely to the direction of transmission of the polarization filter strikes it, then only the component that vibrates parallel to the direction of transmission passes through. The component oscillating perpendicular to the direction of flow is absorbed. An electronic field vector oscillates with light waves.
Polarized light in nature
Light is also partially polarized by reflection on glass, water or a blackboard. Most of the light polarized in the reflection plane is absorbed. The part polarized perpendicular to this plane is transmitted. If light is reflected in the so-called Brewster angle, even complete polarization is possible. Polarized glasses shield this polarized light, which can be valuable when sailing, for example. However, unless they are also darkened, they do not protect against sun rays (especially not against their UV component). The same applies to polarizing filters in cameras.
Polarization can also be achieved by scattering (for example Rayleigh scattering). The light waves hit particles that are much smaller than the wavelength and stimulate them to vibrate. A Hertzian dipole is created.
The (blue) scattered light of the sun in the daytime sky is partially linearly polarized. The polarization is caused by Rayleigh scattering. The plane of polarization is normal to the plane spanned by the line connecting the observer to the sun and the line of sight.
The simplest way to produce polarized light is to let a light beam fall through a polarizing foil. Here, exactly 1 direction of oscillation of the light is filtered out and allowed to pass through the polarization filter (similar directions of oscillation pass through the polarization filter in a weakened manner). So linearly polarized light emerges behind the polarization film. In order to achieve total extinction behind the polarization filter, a second polarizing filter must be inserted behind the first one in such a way that the direction of oscillation of the filter is normal (perpendicular) to that of the first filter (i.e. rotated by 90 °). Now there is no question of the direction of oscillation of the unpolarized light that could penetrate through both polarizing filters. The space behind the 2nd polarization filter is black. The light was extinguished.
At first glance, it is astonishing that a third filter, rotated by 45 °, between the first and the second avoids the cancellation: the light component still present after the first can partly pass the second filter again, since its polarization plane to the second filter yes is only rotated by another 45 °. With optically active substances (liquids: e.g. glucose solution, plastics: e.g. a rapidly cooled geo-triangle, crystals: e.g. quartz) it is possible to rotate the plane of polarization. So it is z. B. possible to achieve a brightening by an optically active substance, if you insert this between two polarizing filters that previously caused extinction (i.e. are perpendicular to each other).
Polarizing filters: Polarization filters used to consist of herapathite crystals (around the middle of the 20th century), but today they are mostly made from polyvinyl alcohol or cellulose hydrate. Other instruments that enable light to be polarized are: the quarter-wave plate (λ / 4 plate), the half-wave plate (λ / 2 plate) as well as various prisms (e.g. the Nicol prism) and their own polarization apparatus.
Polarization of the light can also be caused by scattering, birefringence and reflection.
Analysis of polarized light
Light can be analyzed with the same polarizers that can be used to make polarized light. The polarizers are then called according to their function Analyzers. A pair of linear polarizer and analyzer whose polarization planes are perpendicular to one another does not allow light to pass through. However, there are substances that can rotate the plane of polarization (→ optical activity). I. E. If you place these substances between the polarizer and the analyzer, you can measure the optical activity. Such a setup is called a polarimeter.
Although the sun provides unpolarized light, partially polarized light can also be observed in nature. For example, the scattered light from the blue sky is partially polarized in a linear manner, as is the light reflected on a water surface. Many insects use this effect to orient themselves. Karl von Frisch researched this for the honey bee. In other areas, such as photography, effects generated by polarized light are usually not desired and can be suppressed by using a polarizing filter.
Wilhelm Ritter von Haidinger (1795-1871) described a contrast phenomenon between 1844 and 1854 that appears when looking at a surface illuminated with polarized white (or bluish) light. The blue axis of the phenomenon indicates the direction of the electric field, the yellow axis that of the magnetic field. After a few seconds the appearance fades, but becomes visible again by turning the head. The phenomenon is called Haidinger-Büschel after its discoverer.
Mathematical description of polarization
The polarization state can be described by the four-dimensional real-valued Stokes vectors or by the two-dimensional complex-valued Jones vectors. As an alternative, quasi-monochromatic light can also be described by the coherence matrix. The description of the effect of a polarization-changing optical element is then carried out by multiplication with a corresponding Müller matrix or a Jones matrix.
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