Polarized and natural light. The difference between polarized light and natural

Waves are of two kinds. In longitudinal oscillatory disturbance parallel to the direction of their propagation. An example is the passage of sound in the air. The transverse waves consist of perturbations that are at an angle of 90 ° to the direction of displacement. So, for example, a wave, passing horizontally through the mass of water, causes vertical oscillations on its surface.

The discovery of 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 directions of its oscillations were discovered. The first so-called polarization effect was discovered by the Danish doctor Erasmus Bartolin in 1669. The scientist observed double refraction, or birefringence, in Iceland spar, or calcite (the crystalline form of calcium carbonate). When light passes through calcite, the crystal splits it, producing two images displaced relative to each other.

Newton knew about this phenomenon and suggested that, perhaps, the corpuscles of light have asymmetry or "one-sidedness", which could be the cause of the formation of two images. Huygens, a contemporary of Newton, was able to explain the double refraction of his theory of elementary waves, but he did not understand the true meaning of the effect. Double refraction remained a mystery until Thomas Young and physicist from France Augustin-Jean Fresnel suggested that light waves are 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.

The birefringence is caused by a combination of two perpendicular polarizations, each of which has its own wave velocity. Because of the difference in speed, the two components have different refractive indices, and therefore they are differently refracted through the material, producing two images.

Polarized and natural light: Maxwell's theory

Fresnel quickly developed a complex model of transverse 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 made up of magnetic and electric fields, oscillating perpendicular to the direction of displacement. 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 a right-handed coordinate system. For a wave with frequency f and length λ (they are related 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 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.

Directions of polarization

In describing the orientation of the magnetic and electric fields, light waves are usually indicated only by the direction of the electric field. The vector of the magnetic field is determined by the requirement of 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 latter the fields oscillate in fixed directions as the wave moves.

Other polarization states are possible. In the circular case, the magnetic and electric field vectors rotate relative to the propagation direction with a constant amplitude. Elliptically polarized light is in an intermediate position between linear and circular polarizations.

Non-polarized light

Atoms on the surface of a heated filament that generate electromagnetic radiation operate independently of one another. Each radiation can be approximately modeled in the form of short trains lasting from 10 ^-9 to 10 ^-8 seconds. The electromagnetic wave emanating from the 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 varies 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 lies in the fact that in the first one the oscillations take place in the same plane.

Sources of polarized radiation

Polarized light can be produced in cases where the 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 known astronomical sources that emit naturally polarized light. These include nebulae, supernova remnants and active galactic nuclei. The polarization of cosmic radiation is 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 being the polaroid created by the American physicist Edwin Land. The filter consists of long chains of hydrocarbon molecules oriented in one direction through a heat treatment process. Molecules selectively absorb radiation, whose electric field is parallel to their orientation. The light emerging from the polaroid is linearly polarized. Its electric field is perpendicular to the orientation of the molecules. Polaroid has found application in many areas, including sunglasses and light filters, which reduce the effect of reflected and scattered light.

Natural and polarized light: the law of Malus

In 1808, the physicist Etienne-Louis Malius 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 of the extreme cases, when the tangent of the angle of incidence of the ray 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). Direction of polarization parallel to the reflecting surface. Since daylight glare, as a rule, occurs when reflected from horizontal surfaces, such as roads and water, sunglasses often use filters to remove horizontally polarized light and, therefore, selectively remove glints of light.

Rayleigh scattering

The scattering of light by very small objects, the dimensions of which are much smaller than the wavelength (the so-called Rayleigh scattering by the name of the English scientist Lord Rayleigh), also creates a partial polarization. When solar radiation passes through the earth's atmosphere, it is dissipated by air molecules. The Earth reaches a 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, remains unnoticed. Nevertheless, the eyes of many insects respond to it, and they use the relative polarization of scattered radiation as a navigation tool. A conventional camera light filter, used to reduce background radiation in bright sunlight, is a simple linear polarizer that separates the natural and polarized Rayleigh 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, certain biological structures and optically active materials. The technological application includes polarization microscopes, liquid crystal displays and optical instruments used for material research.