Waves come in two forms. In longitudinal vibrational disturbance parallel to the direction of their propagation. An example is the passage of sound in air. Transverse waves consist of perturbations that are at an angle of 90 ° to the direction of movement. So, for example, a wave passing horizontally through a mass of water causes vertical oscillations on its surface.
Discovery of the phenomenon
A number of mysterious optical effects observed in the middle of the 17th century were explained when polarized and natural light began to be regarded as a wave phenomenon and the directions of its oscillations were discovered. The first so-called polarization effect was discovered by Danish physician Erasmus Bartolin in 1669. The scientist observed double refraction, or birefringence, in Icelandic spar, or calcite (a crystalline form of calcium carbonate). When light passes through calcite, the crystal splits it, producing two images that are offset from each other.
Newton was aware of this phenomenon and suggested that perhaps the light corpuscles have an asymmetry or “one-sidedness”, which could be the reason for the formation of two images. Huygens, a contemporary of Newton, was able to explain the double refraction by his theory of elementary waves, but he did not understand the true meaning of the effect. Birefringence remained a mystery until Thomas Jung and French physicist Augustin-Jean-Fresnel suggested that the light waves were transverse. A simple idea made it possible to explain what polarized and natural light is. This provided a natural and uncomplicated basis for the analysis of polarization effects.
Birefringence is caused by a combination of two perpendicular polarizations, each of which has its own wave velocity. Due to the difference in speed, the two components have different refractive indices, and therefore they are refracted differently through the material, producing two images.
Polarized and natural light: Maxwell's theory
Fresnel quickly developed a comprehensive model of shear waves, which led to birefringence and a number of other optical effects. Forty years later , Maxwell's electromagnetic theory elegantly explained the transverse nature of light.
Maxwell’s electromagnetic waves are composed of magnetic and electric fields that vibrate perpendicular to the direction of travel. The fields are at an angle of 90 ° to each other. In this case, the directions of propagation of the magnetic and electric fields form the right coordinate system. For a wave with frequency f and length λ (they are connected by the dependence λf = c ), which moves in the positive x direction, the fields are described mathematically:
- E (x, t) = E 0 cos (2 π x / λ - 2 π ft) y ^;
- B (x, t) = B 0 cos (2 π x / λ - 2 π ft) z ^.
The equations show that the electric and magnetic fields are in phase with each other. At any given moment in time, they simultaneously reach their maximum values in space, equal to E 0 and B 0 . These amplitudes are not independent. Maxwell's equations show that E 0 = cB 0 for all electromagnetic waves in a vacuum.
Polarization directions
In describing the orientation of the magnetic and electric fields, light waves usually indicate only the direction of the electric field. The magnetic field vector is determined by the requirement of the perpendicularity of the fields and their perpendicularity to the direction of motion. Natural and linearly polarized light is distinguished by the fact that in the last field they oscillate in fixed directions as the waves move.
Other polarization states are also possible. In the case of circular vectors of the magnetic and electric fields rotate relative to the direction of propagation with a constant amplitude. Elliptically polarized light is in an intermediate position between linear and circular polarizations.
Unpolarized light
Atoms on the surface of a heated filament that generate electromagnetic radiation act independently of each other. Each radiation can be approximately modeled as short trains lasting from 10 -9 to 10 -8 seconds. An electromagnetic wave emanating from an incandescent filament is a superposition of these trains, each of which has its own direction of polarization. The sum of randomly oriented trains forms a wave whose polarization vector changes rapidly and randomly. Such a wave is called unpolarized. All natural light sources, including the Sun, incandescent lamps, fluorescent lamps and flames, produce such radiation. However, natural light is often partially polarized due to multiple scattering and reflection.
Thus, the difference between polarized light and natural light is that in the first, the vibrations occur in the same plane.
Sources of Polarized Radiation
Polarized light can be produced when spatial orientation is determined. One example is synchrotron radiation, in which high-energy charged particles move in a magnetic field and emit polarized electromagnetic waves. There are many well-known astronomical sources that emit naturally polarized light. These include nebulae, supernova remnants, and active galactic nuclei. The polarization of cosmic radiation is being studied in order to determine the properties of its sources.
Polaroid Filter
Polarized and natural light are separated when passing through a series of materials, the most common of which is the polaroid created by the American physicist Edwin Land. The filter consists of long chains of hydrocarbon molecules oriented in one direction by a heat treatment process. Molecules selectively absorb radiation whose electric field is parallel to their orientation. Light emerging from a polaroid is linearly polarized. Its electric field is perpendicular to the direction of orientation of the molecules. Polaroid has found application in many areas, including sunglasses and filters, which reduce the effect of reflected and scattered light.
Natural and polarized light: the law of Malus
In 1808, physicist Etienne-Louis Malus discovered that light reflected from non-metallic surfaces is partially polarized. The degree of this effect depends on the angle of incidence and the refractive index of the reflecting material. In one extreme case, when the tangent of the angle of incidence of the beam in the air is equal to the refractive index of the reflecting material, the reflected light becomes completely linearly polarized. This phenomenon is known as Brewster’s law (named after its discoverer, Scottish physicist David Brewster). The direction of polarization is parallel to the reflective surface. Since daylight glare tends to occur when reflected from horizontal surfaces such as roads and water, filters are often used in sunglasses to remove horizontally polarized light and, therefore, selectively remove light reflections.
Rayleigh scattering
The scattering of light by very small objects, whose dimensions are much smaller than the wavelength (the so-called Rayleigh scattering on behalf of the English scientist Lord Rayleigh), also creates partial polarization. When solar radiation passes through the earth's atmosphere, it is scattered by air molecules. Earth reaches diffused polarized and natural light. The degree of its polarization depends on the scattering angle. Since a person does not distinguish between natural and polarized light, this effect, as a rule, goes unnoticed. Nevertheless, the eyes of many insects react to it, and they use the relative polarization of the scattered radiation as a navigation tool. A conventional camera filter, used to reduce background radiation in bright sunlight, is a simple linear polarizer that separates Rayleigh’s natural and polarized light.
Anisotropic materials
Polarization effects are observed in optically anisotropic materials (in which the refractive index varies with the direction of polarization), such as birefringent crystals, some biological structures, and optically active materials. Technological applications include polarizing microscopes, liquid crystal displays and optical instruments used to study materials.