Why polarization is important

Mirrors are not good polarizers, although a wide spectrum of transparent materials act as very good polarizers, but only if the incident light angle is oriented within certain limits. An important property of reflected polarized light is that the degree of polarization is dependent upon the incident angle of the light, with the increasing amounts of polarization being observed for decreasing incident angles.

When considering the incidence of non-polarized light on a flat insulating surface, there is a unique angle at which the reflected light waves are all polarized into a single plane. This angle is commonly referred to as Brewster's angle , and can be easily calculated utilizing the following equation for a beam of light traveling through air:. By examining the equation, it becomes obvious that the refractive index of an unknown specimen can be determined by the Brewster angle.

This feature is particularly useful in the case of opaque materials that have high absorption coefficients for transmitted light, rendering the usual Snell's Law formula inapplicable. Determining the amount of polarization through reflection techniques also eases the search for the polarizing axis on a sheet of polarizing film that is not marked. The principle behind Brewster's angle is illustrated Figure 3 for a single ray of light reflecting from the flat surface of a transparent medium having a higher refractive index than air.

The incident ray is drawn with only two electric vector vibration planes, but is intended to represent light having vibrations in all planes perpendicular to the direction of propagation. The incidence plane is defined by the incident, refracted, and reflected waves. The refracted ray is oriented at a degree angle from the reflected ray and is only partially polarized.

For water refractive index of 1. Light reflected from a highway surface at the Brewster angle often produces annoying and distracting glare , which can be demonstrated quite easily by viewing the distant part of a highway or the surface of a swimming pool on a hot, sunny day. Modern lasers commonly take advantage of Brewster's angle to produce linearly polarized light from reflections at the mirrored surfaces positioned near the ends of the laser cavity.

As discussed above, bright reflections originating from horizontal surfaces, such as the highway or the water in a pool, are partially polarized with the electric field vectors vibrating in a direction that is parallel to the ground. This light can be blocked by polarizing filters oriented in a vertical direction, as illustrated in Figure 4, with a pair of polarized sunglasses.

The lenses of the sunglasses have polarizing filters that are oriented vertically with respect to the frames. In the figure, the blue light waves have their electric field vectors oriented in the same direction as the polarizing lenses and, thus, are passed through. In contrast, the red light wave vibration orientation is perpendicular to the filter orientation and is blocked by the lenses.

Polarizing sunglasses are very useful when driving in the sun or at the beach where sunlight is reflected from the surface of the road or water, leading to glare that can be almost blinding. Polarizing filters are also quite useful in photography, where they can be attached to the front of a camera lens to reduce glare and increase overall image contrast in photographs or digital images.

Polarizers utilized on cameras are generally designed with a mounting ring that allows them to be rotated in use to achieve the desired effect under various lighting conditions.

Arago investigated the polarity of light originating from various sources in the sky and proposed a theory that predicted the velocity of light should decrease as it passes into a denser medium. He also worked with Augustin Fresnel to investigate interference in polarized light and discovered that two beams of light polarized with their vibration directions oriented perpendicular to each other will not undergo interference. Arago's polarizing filters, designed and built in , were made from a stack of glass sheets pressed together.

A majority of the polarizing materials used today are derived from synthetic films invented by Dr. Edwin H. Land in , which soon overtook all other materials as the medium of choice for production of plane-polarized light. To produce the films, tiny crystallites of iodoquinine sulfate, oriented in the same direction, are embedded in a transparent polymeric film to prevent migration and reorientation of the crystals.

Land developed sheets containing polarizing films that are marketed under the trade name of Polaroid a registered trademark , which has become the accepted generic term for these sheets. Any device capable of selecting plane-polarized light from natural non-polarized white light is now referred to as a polar or polarizer , a name first introduced in by A. Because these filters are capable of differentially transmitting light rays, depending upon their orientation with respect to the polarizer axis, they exhibit a form of dichroism , and are often termed dichroic filters.

Polarized light microscopy was first introduced during the nineteenth century, but instead of employing transmission-polarizing materials, light was polarized by reflection from a stack of glass plates set at a degree angle to the plane of incidence.

Later, more advanced instruments relied on a crystal of doubly refracting material such as calcite specially cut and cemented together to form a prism. A beam of white non-polarized light entering a crystal of this type is separated into two components that are polarized in mutually perpendicular orthogonal directions. One of the light rays emerging from a birefringent crystal is termed the ordinary ray , while the other is called the extraordinary ray.

The ordinary ray is refracted to a greater degree by electrostatic forces in the crystal and impacts the cemented surface at the critical angle of total internal reflection. As a result, this ray is reflected out of the prism and eliminated by absorption in the optical mount. The extraordinary ray traverses the prism and emerges as a beam of linearly-polarized light that is passed directly through the condenser and to the specimen positioned on the microscope stage.

Several versions of prism-based polarizing devices were once widely available, and these were usually named after their designers. The most common polarizing prism illustrated in Figure 5 was named after William Nicol, who first cleaved and cemented together two crystals of Iceland spar with Canada balsam in Nicol prisms were first used to measure the polarization angle of birefringent compounds, leading to new developments in the understanding of interactions between polarized light and crystalline substances.

Presented in Figure 5 is an illustration of the construction of a typical Nicol prism. A crystal of doubly refracting birefringent material, usually calcite, is cut along the plane labeled a-b-c-d and the two halves are then cemented together to reproduce the original crystal shape. A beam of non-polarized white light enters the crystal from the left and is split into two components that are polarized in mutually perpendicular directions.

One of these beams labeled the ordinary ray is refracted to a greater degree and impacts the cemented boundary at an angle that results in its total reflection out of the prism through the uppermost crystal face. The other beam extraordinary ray is refracted to a lesser degree and passes through the prism to exit as a plane-polarized beam of light.

In fact, for the example illustrated above, the particular choice of L for a given difference between n x and n y causes the linearly polarized light at the input end to be converted to circularly polarized light at the other end of the birefringent material.

How did this happen? Consider the phases accumulated by the two component waves as they travel through the birefringent material. This occurs when or when Because of this relationship, a material with birefringence Dn of the appropriate thickness L to convert linear polarization to circular polarization is called a quarter-wave plate.

What causes materials to be birefringent? Some materials, especially crystals, are naturally anisotropic at microscopic sub-wavelength size scales. For example, Calcite CaCO 3 is shown in the drawing below. The structure, and hence the response to polarized light, along the c direction is markedly different than that along the a and b directions, thus leading to a different index of refraction for light polarized along this direction.

Other materials are nominally isotropic, but when they are bent or deformed in some way, they become anisotropic and therefore exhibit birefringence. This effect is widely used to study the mechanical properties of materials with optics. Effects of reflection and transmission on polarization The polarization of light reflected and transmitted at an interface between two media or at a thin-film multilayer coating can be altered dramatically.

These two cases are considered below. Polarization dependence of light reflected or transmitted at an interface When light is incident on an interface between two different media with different indexes of refraction, some of the light is reflected and some is transmitted. When the angle of incidence is not normal, different polarizations are reflected and transmitted by different amounts.

In the example in the diagram below, the plane of incidence is the plane containing the x and z axes. That is, E s y , while E p lies in the x-z plane.

It turns out that s-polarized light is always more highly reflected than p-polarized light. The power or intensity reflection coefficients for a light wave i. The Fresnel reflection coefficients for non-normal incidence are given by the equations. For angles of incidence below the critical angle only the amplitudes of the different polarization components are affected by reflection or transmission at an interface.

Thus, the state of polarization can change in only limited ways. For example, linearly polarized light remains linearly polarized, although its orientation angle may rotate. Thus linearly polarized light may become elliptical, or vice versa, in addition to changes in the orientation.

Commercial polarizing beam splitters PBS from different vendors use many different designs to achieve their high performance in a sweet spot of wavelength and angle. Systems which meet specification with one PBS have had yield problems and even failed when a different PBS was substituted.

One very expensive polarization-related failure occurred in a gimballed mirror for a satellite-to-submarine communication system. No other orientations were checked, based on the false assumption that arbitrary light can just be analyzed as a combination of x and y components. A Polaris-M polarization analysis with Mueller matrices and the specified coatings would have quickly revealed the large diattenuation and retardance in the coating choice and led to a coating change during the design.

Instead the program lost 18 months and costs increased by over two million dollars. Mass Spectrometers. MKS Instruments. Compare All. Resources T utorials Optics Introduction to Polarizers. Polarization States. Figure 1. Depiction of a linearly polarized wave left and standard symbols for linearly polarized light right. Circular Polarization.

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