Why is scattered light polarized

Those rays that have angles close to parallel with respect to the crystal axis will be adsorbed to a much greater degree than those having angles close to the perpendicular.

The most common Polaroid filters termed the H-series transmit only about 25 percent of the incident light beam, but the degree of polarization of the transmitted rays exceeds 99 percent. A number of applications, most notably polarized optical microscopy, rely on crossed polarizers to examine birefringent or doubly refracting specimens.

When two polarizers are crossed, their transmission axes are oriented perpendicular to each other and light passing through the first polarizer is completely extinguished, or absorbed, by the second polarizer, which is typically termed an analyzer.

The light-absorbing quality of a dichroic polarizing filter determines exactly how much random light is extinguished when the polarizer is utilized in a crossed pair, and is referred to as the extinction factor of the polarizer. Quantitatively, the extinction factor is determined by the ratio of light that is passed by a pair of polarizers when their transmission axes are oriented parallel versus the amount passed when they are positioned perpendicular to each other.

In general, extinction factors between 10, and , are required to produce jet-black backgrounds and maximum observable specimen birefringence and contrast in polarized optical microscopy. The amount of light passing through a crossed pair of high-quality polarizers is determined by the orientation of the analyzer with respect to the polarizer.

When the polarizers are oriented perpendicular to each other, they display a maximum level of extinction. However, at other angles, varying degrees of extinction are obtained, as illustrated by the vector diagrams presented in Figure 6. The analyzer is utilized to control the amount of light passing through the crossed pair, and can be rotated in the light path to enable various amplitudes of polarized light to pass through. In Figure 6 a , the polarizer and analyzer have parallel transmission axes and the electric vectors of light passing through the polarizer and analyzer are of equal magnitude and parallel to each other.

Rotating the analyzer transmission axis by degrees with respect to that of the polarizer reduces the amplitude of a light wave passing through the pair, as illustrated in Figure 6 b. In this case, the polarized light transmitted through the polarizer can be resolved into horizontal and vertical components by vector mathematics to determine the amplitude of polarized light that is able to pass through the analyzer.

The amplitude of the ray transmitted through the analyzer is equal to the vertical vector component illustrated as the yellow arrow in Figure 6 b. Continued rotation of the analyzer transmission axis, to a degree angle with respect to the transmission axis of the polarizer, further reduces the magnitude of the vector component that is transmitted through the analyzer Figure 6 c.

When the analyzer and polarizer are completely crossed degree angle , the vertical component becomes negligible Figure 6 d and the polarizers have achieved their maximum extinction value.

The amount of light passing through a pair of polarizers can be quantitatively described by applying Malus' cosine-squared law, as a function of the angles between the polarizer transmission axes, utilizing the equation:. In this case, light passed by the polarizer is completely extinguished by the analyzer.

When the polarizers are partially crossed at 30 and 60 degrees, the light transmitted by the analyzer is reduced by 25 percent and 75 percent, respectively. Gas and water molecules in the atmosphere scatter light from the sun in all directions, an effect that is responsible for blue skies, white clouds, red sunsets, and a phenomenon termed atmospheric polarization. The amount of light scattered termed Rayleigh scattering depends upon the size of the molecules hydrogen, oxygen, water and the wavelength of light, as demonstrated by Lord Rayleigh in Longer wavelengths, such as red, orange, and yellow, are not scattered as effectively as are the shorter wavelengths, such as violet and blue.

Atmospheric polarization is a direct result of the Rayleigh scattering of sunlight by gas molecules in the atmosphere. Upon impact between a photon from the sun and a gas molecule, the electric field from the photon induces a vibration and subsequent re-radiation of polarized light from the molecule illustrated in Figure 7.

The radiated light is scattered at right angles to the direction of sunlight propagation, and is polarized either vertically or horizontally, depending upon the direction of scatter. A majority of the polarized light impacting the Earth is polarized horizontally over 50 percent , a fact that can be confirmed by viewing the sky through a Polaroid filter. Reports have surfaced that certain species of insects and animals are able to detect polarized light, including ants, fruit flies, and certain fish, although the list may actually be much longer.

For example, several insect species primarily honeybees are thought to employ polarized light in navigating to their destinations. It is also widely believed that some individuals are sensitive to polarized light, and are able to observe a yellow horizontal line superimposed on the blue sky when staring in a direction perpendicular to the sun's direction a phenomenon termed Haidinger's brush. Yellow pigment proteins, termed macula lutea , which are dichroic crystals residing in the fovea of the human eye, are credited with enabling a person to view polarized light.

In linearly polarized light, the electric vector is vibrating in a plane that is perpendicular to the direction of propagation, as discussed above. Natural light sources, such as sunlight, and artificial sources, including incandescent and fluorescent light, all emit light with orientations of the electric vector that are random in space and time.

Light of this type is termed non-polarized. In addition, there exist several states of elliptically polarized light that lie between linear and non-polarized, in which the electric field vector transcribes the shape of an ellipse in all planes perpendicular to the direction of light wave propagation. Elliptical polarization, unlike plane-polarized and non-polarized light, has a rotational "sense" that refers to the direction of electric vector rotation around the propagation incident axis of the light beam.

When viewed end-on, the direction of polarization can be either left-handed or right-handed, a property that is termed the handedness of the elliptical polarization. Clockwise rotational sweeps of the vector are referred to as right-handed polarization, and counterclockwise rotational sweeps represent left-handed polarization.

In cases where the major and minor vectorial axes of the polarization ellipse are equal, then the light wave falls into the category of circularly polarized light, and can be either right-handed or left-handed in sense. Another case often occurs in which the minor axis of the electric vector component in elliptically polarized light goes to zero, and the light becomes linearly polarized. Although each of these polarization motifs can be achieved in the laboratory with the appropriate optical instrumentation, they also occur to varying, but minor, degrees in natural non-polarized light.

The ordinary and extraordinary light waves generated when a beam of light traverses a birefringent crystal have plane-polarized electric vectors that are mutually perpendicular to each other. In addition, due to differences in electronic interaction that each component experiences during its journey through the crystal, a phase shift usually occurs between the two waves.

Although the ordinary and extraordinary waves follow separate trajectories and are widely separated in the calcite crystal described previously, this is not usually the case for crystalline materials having an optical axis that is perpendicular to the plane of incident illumination. A special class of materials, known as compensation or retardation plates, are quite useful in producing elliptically and circularly polarized light for a number of applications, including polarized optical microscopy.

These birefringent substances are chosen because, when their optical axis is positioned perpendicular to the incident light beam, the ordinary and extraordinary light rays follow identical trajectories and exhibit a phase difference that is dependent upon the degree of birefringence.

Because the pair of orthogonal waves is superimposed, it can be considered a single wave having mutually perpendicular electrical vector components separated by a small difference in phase. When the vectors are combined by simple addition in three-dimensional space, the resulting wave becomes elliptically polarized. This concept is illustrated in Figure 8 , where the resultant electric vector does not vibrate in a single plane, but progressively rotates around the axis of light wave propagation, sweeping out an elliptical trajectory that appears as a spiral when the wave is viewed at an angle.

The size of the phase difference between the ordinary and extraordinary waves of equal amplitude determines whether the vector sweeps an elliptical or circular pathway when the wave is viewed end-on from the direction of propagation.

If the phase shift is either one-quarter or three-quarters of a wavelength, then a circular spiral is scribed by the resultant vector. However, phase shifts of one-half or a full wavelength produce linearly polarized light, and all other phase shifts produce sweeps having various degrees of ellipticity.

When the ordinary and extraordinary waves emerge from a birefringent crystal, they are vibrating in mutually perpendicular planes having a total intensity that is the sum of their individual intensities. Because the polarized waves have electric vectors that vibrate in perpendicular planes, the waves are not capable of undergoing interference. This fact has consequences in the ability of birefringent substances to produce an image.

Interference can only occur when the electric vectors of two waves vibrate in the same plane during intersection to produce a change in amplitude of the resultant wave a requirement for image formation. Therefore, transparent specimens that are birefringent will remain invisible unless they are examined between crossed polarizers, which pass only the components of the elliptically and circularly polarized waves that are parallel to the axis of the polarizer closest to the observer.

These components are able to produce amplitude fluctuations to generate contrast and emerge from the polarizer as linearly polarized light. One of the most common and practical applications of polarization is the liquid crystal display LCD used in numerous devices including wristwatches, computer screens, timers, clocks, and a host of others.

These display systems are based upon the interaction of rod-like liquid crystalline molecules with an electric field and polarized light waves.

The liquid crystalline phase exists in a ground state that is termed cholesteric , in which the molecules are oriented in layers, and each successive layer is slightly twisted to form a spiral pattern Figure 9.

When polarized light waves interact with the liquid crystalline phase the wave is "twisted" by an angle of approximately 90 degrees with respect to the incident wave. The exact magnitude of this angle is a function of the helical pitch of the cholesteric liquid crystalline phase, which is dependent upon the chemical composition of the molecules it can be fine-tuned by small changes to the molecular structure.

It can sit on the cart too, but leave a small distance between it and the tank so you can insert the Polaroid.

The unscattered light should fall onto a projection screen several meters ahead of the cart, angled so that the audience can see the spot. Be careful not to add too much milk, as multiple scattering will occur and wash out the color effect as well as the polarization.

Complete key to listings. Skip to main content. Main Menu Utility Menu Search. Browse Catalog. What it shows: Unpolarized light passing through a fluid is scattered; the scattered light being partially or completely plane polarized. How it works: Unpolarized white light from a slide projector enters a fish tank of very slightly milky water. Most light sources produce unpolarized light. When light hits a reflective surface, the vertically polarized aspects of that light are refracted at that surface.

The reflected light is more horizontally polarized. To better remember this, we can think of light as an arrow and the reflective surface as a target. If the arrow hits the target perpendicularly vertically polarized , it is going to stick in the target be refracted into the surface. If the arrow hits the target on its side horizontally polarized then it will bounce right off be reflected.

Unpolarized light has equal amounts of vertical and horizontal polarization. After interaction with a surface, the vertical components are preferentially absorbed or refracted, leaving the reflected light more horizontally polarized. This is akin to arrows striking on their sides bouncing off, whereas arrows striking on their tips go into the surface.

Since the light is split into two, and part of it is refracted, the amount of polarization to the reflected light depends on the index of refraction of the reflective surface. We can use the following equation to determine the angle of reflection at which light will be completely polarized:.

Just as unpolarized light can be partially polarized by reflecting, it can also be polarized by scattering also known as Rayleigh scattering; illustrated in. Since light waves are electromagnetic EM waves and EM waves are transverse waves they will vibrate the electrons of air molecules perpendicular to the direction in which they are traveling.

The electrons then produce radiation acting like small antennae that is polarized perpendicular to the direction of the ray.