How is polarized light used

But if you shake it up and down you create a vertically polarised wave. Generally, light is a mixture of polarisations, but sometimes — for example in parts of the sky, on your computer screen and in reflections from water or glass — a large percentage of the waves are oscillating in the same orientation. This light is described as being strongly polarised. You will probably have come across technology that is built around polarised light before. This is possible because light reflected into our eyes from horizontal surfaces is horizontally polarised and the sunglasses have a structure like a picket fence, so they only let in vertically polarised oscillations, blocking out the horizontally polarised bright reflections.

Polarised light is at the heart of modern 3D cinema and LCD computer screens, smart phones and tablets. You may also see a bluish bow-tie at right angles to the yellow one.

This is one of the reasons that few people notice them day-to-day, and why they have previously been fairly difficult to study. Our results shows that your cornea can dramatically affect how you perceive polarised light. Tilt your head from side to side and faint yellow and blue bow-ties, slightly larger than your thumb, should become visible.

There are specific directions for the oscillations of the electric and magnetic fields. This is not the same type of polarization as that discussed for the separation of charges. Waves having such a direction are said to be polarized.

For an EM wave, we define the direction of polarization to be the direction parallel to the electric field. Thus we can think of the electric field arrows as showing the direction of polarization, as in Figure 2. To examine this further, consider the transverse waves in the ropes shown in Figure 3. The oscillations in one rope are in a vertical plane and are said to be vertically polarized.

Those in the other rope are in a horizontal plane and are horizontally polarized. If a vertical slit is placed on the first rope, the waves pass through. However, a vertical slit blocks the horizontally polarized waves.

For EM waves, the direction of the electric field is analogous to the disturbances on the ropes. Figure 3. The transverse oscillations in one rope are in a vertical plane, and those in the other rope are in a horizontal plane. The first is said to be vertically polarized, and the other is said to be horizontally polarized.

Vertical slits pass vertically polarized waves and block horizontally polarized waves. Figure 4. The slender arrow represents a ray of unpolarized light. The bold arrows represent the direction of polarization of the individual waves composing the ray. Since the light is unpolarized, the arrows point in all directions. The Sun and many other light sources produce waves that are randomly polarized see Figure 4.

Such light is said to be unpolarized because it is composed of many waves with all possible directions of polarization.

Polaroid materials, invented by the founder of Polaroid Corporation, Edwin Land, act as a polarizing slit for light, allowing only polarization in one direction to pass through. Polarizing filters are composed of long molecules aligned in one direction. Thinking of the molecules as many slits, analogous to those for the oscillating ropes, we can understand why only light with a specific polarization can get through. The axis of a polarizing filter is the direction along which the filter passes the electric field of an EM wave see Figure 5.

Figure 5. A polarizing filter has a polarization axis that acts as a slit passing through electric fields parallel to its direction. The direction of polarization of an EM wave is defined to be the direction of its electric field.

Figure 6 shows the effect of two polarizing filters on originally unpolarized light. The first filter polarizes the light along its axis. When the axes of the first and second filters are aligned parallel , then all of the polarized light passed by the first filter is also passed by the second. When the axes are perpendicular, no light is passed by the second.

Figure 6. The effect of rotating two polarizing filters, where the first polarizes the light. Its axis is perpendicular to the filter on the right dark area and parallel to the filter on the left lighter area. Figure 7. A polarizing filter transmits only the component of the wave parallel to its axis, , reducing the intensity of any light not polarized parallel to its axis. Only the component of the EM wave parallel to the axis of a filter is passed.

What angle is needed between the direction of polarized light and the axis of a polarizing filter to reduce its intensity by When the intensity is reduced by A fairly large angle between the direction of polarization and the filter axis is needed to reduce the intensity to This seems reasonable based on experimenting with polarizing films.

Note that By now you can probably guess that Polaroid sunglasses cut the glare in reflected light because that light is polarized. You can check this for yourself by holding Polaroid sunglasses in front of you and rotating them while looking at light reflected from water or glass. As you rotate the sunglasses, you will notice the light gets bright and dim, but not completely black. This implies the reflected light is partially polarized and cannot be completely blocked by a polarizing filter.

Figure 8. Polarization by reflection. 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.

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. Photographers know that this partial polarization of scattered light leads to photographs characterized by a washed-out sky.

The problem can easily be corrected by the use of a Polaroid filter. As the filter is rotated, the partially polarized light is blocked and the glare is reduced. The photographic secret of capturing a vivid blue sky as the backdrop of a beautiful foreground lies in the physics of polarization and Polaroid filters. Polarization has a wealth of other applications besides their use in glare-reducing sunglasses.

In industry, Polaroid filters are used to perform stress analysis tests on transparent plastics. As light passes through a plastic, each color of visible light is polarized with its own orientation. If such a plastic is placed between two polarizing plates, a colorful pattern is revealed. As the top plate is turned, the color pattern changes as new colors become blocked and the formerly blocked colors are transmitted. A common Physics demonstration involves placing a plastic protractor between two Polaroid plates and placing them on top of an overhead projector.

It is known that structural stress in plastic is signified at locations where there is a large concentration of colored bands. This location of stress is usually the location where structural failure will most likely occur. Perhaps you wish that a more careful stress analysis were performed on the plastic case of the CD that you recently purchased.

Polarization is also used in the entertainment industry to produce and show 3-D movies. Three-dimensional movies are actually two movies being shown at the same time through two projectors.

The two movies are filmed from two slightly different camera locations. Each individual movie is then projected from different sides of the audience onto a metal screen. The movies are projected through a polarizing filter. The polarizing filter used for the projector on the left may have its polarization axis aligned horizontally while the polarizing filter used for the projector on the right would have its polarization axis aligned vertically. Consequently, there are two slightly different movies being projected onto a screen.

Each movie is cast by light that is polarized with an orientation perpendicular to the other movie. The audience then wears glasses that have two Polaroid filters. Each filter has a different polarization axis - one is horizontal and the other is vertical. The result of this arrangement of projectors and filters is that the left eye sees the movie that is projected from the right projector while the right eye sees the movie that is projected from the left projector.

This gives the viewer a perception of depth. Our model of the polarization of light provides some substantial support for the wavelike nature of light. It would be extremely difficult to explain polarization phenomenon using a particle view of light.

Polarization would only occur with a transverse wave. For this reason, polarization is one more reason why scientists believe that light exhibits wavelike behavior. Suppose that light passes through two Polaroid filters whose polarization axes are parallel to each other. What would be the result? The first filter will polarize the light, blocking one-half of its vibrations. The second filter will have no affect on the light.

Being aligned parallel to the first filter, the second filter will let the same light waves through. Light becomes partially polarized as it reflects off nonmetallic surfaces such as glass, water, or a road surface.