Polarization Of Light

Type of physical science: Classical physics

Field of study: Optics

One consequence of the wave properties of light is polarization. The polarization state defines the directions in which the electric and magnetic fields, which define light, oscillate.

Overview

Certain properties of light are best described by a model depicting light as a stream of particles, moving at the speed of light, which is about 300,000 kilometers per second. These light particles are called photons, and this model is referred to as the "corpuscular" model, indicating that there are discrete particles, or corpuscles, of light. The other model to explain many properties of light depicts light as a wave. In this model, the propagation media are the electric and magnetic fields. Light waves can travel through a vacuum, as well as through media, whereas sound waves, for example, require gaseous, liquid, or solid media for propagation.

This dualism in the description of light, also known as wave-particle duality, is one of the most fascinating aspects of science: Light exhibits behavior that is explicable only in terms of its wave properties (for example, diffraction, refraction, and reflection), and also exhibits particular properties, such as the photoelectric effect.

One property that is understood most easily from a wave description of light is the concept of polarization. Nevertheless, in order to describe the concept of polarization, it is necessary to define how light propagates through space. Visible light is a form of electromagnetic radiation. In fact, visible light constitutes only a small range of the electromagnetic spectrum, which extends from the γ-ray and X-ray region into the very longwave radio region, known as longwave radiation.

Any form of electromagnetic radiation can be described as an alternating electric and magnetic field. Figure 1a shows the alternating electric field of electromagnetic radiation. Here the propagation direction of the radiation is along the x-axis, and the electric field E oscillates in the plane of the page. The magnetic field B, which is not shown in the figure, varies in phase with the electric field, but in a plane perpendicular to the plane of the page. Since E and B are always in phase with and perpendicular to each other, it is customary to depict light in terms of the electric field only. Figure 1a also shows the wavelength λ, which measures the distance along the x-axis, between two consecutive crests of the wave. In electromagnetic radiation, the oscillation of the electric field occurs perpendicular to the propagation direction of the light; that is, the electric field varies (or is "polarized along") along the y-axis, whereas the light wave propagates along the x-axis. Thus, light is referred to as a transversal wave.

Plane-polarized radiation is light in which the electric field oscillates exclusively in one plane, as shown in figure 1a. An observer at position O would see the electric vector oscillate in one line parallel to the y-axis (shown in figure 1b); therefore, plane-polarized light is also referred to as linearly polarized light. Linearly polarized light can be created naturally during certain reflection processes; however, most forms of visible light created spontaneously are not linearly polarized, but randomly polarized. Randomly polarized light is created when randomly aligned atoms or molecules--such as the metal atoms in a hot filament found in a light bulb or the gaseous atoms in a fluorescent light bulb--emit light. In randomly polarized light, all possible directions of the electric vector perpendicular to the x-axis occur with equal probability; thus, an observer at point O would see the electric vectors, as shown in figure 1c. Linearly polarized light is actually a special case of randomly polarized light, where only one component is selected from all possible directions.

Selection of one directional component may be achieved in a number of ways. One of the methods to polarize light is by reflection from nonmetallic surfaces. This method is shown schematically in figure 2. Here, the beam of light (I) incident on the surface is randomly polarized, as indicated by the direction of the electric vectors. If the angle of incidence of the light beam onto the surface is chosen properly, the light beam (RE) reflected from the surface will be plane-polarized, whereas the refracted beam (RA) is only partially polarized. The reflected beam is polarized with the electric vector parallel to the plane of the reflecting surface.

The angle of incidence, at which the reflected light is completely linearly polarized, is called Brewster's angle (named for Sir David Brewster) and depends on the nature of the reflecting material and the color of light. Brewster's angle is about 57 degrees for a glass-air interface; thus, light reflected from glass at this angle will be linearly polarized.

The principles of polarization by reflection can be understood quite easily in a conceptual manner. For this purpose, the reflection process must be visualized as follows: An incident wave sets up an oscillation of atoms and electrons in the surface of the reflecting material; these oscillating particles reradiate the reflected wave. Only light polarized in the plane of the reflecting surface can set up these oscillations; at Brewster's angle, the complete polarization of the reflected beam is achieved. Furthermore, the refracted beam is most intense if the angle of incidence is Brewster's angle.

Most commercially available linear polarizers operate on different principles. The polarization filters, used in photography or simple optical demonstrations, are made of a stretched film of material that absorbs light. By stretching the film, it is possible to align the molecules such that they are approximately parallel. Single molecules, or molecules aligned in such a way, have the property of completely absorbing one direction of polarization and transmitting the other with minimal loss. This property is called linear dichroism. The direction of the polarizing film, which is parallel to the plane of polarization of the emerging light, is called the polarization axis. These sheet polarizers, or Polaroid sheets, are inexpensive and rugged and are often found in elementary optics experiments.

Another method to create linearly polarized light uses a phenomenon known as double refraction. Double refraction occurs in certain minerals, such as quartz or calcite, which are known to be birefringent (light is refracted in two directions to form two rays). When a beam of light impinges onto a slab of calcite, two beams of light are refracted into the crystal. These two beams are called the ordinary and extraordinary beams. Depending on the angles between the crystal axes and the light beam, the two refracted beams can be polarized completely. Thus, birefringence resolves the incident light into two beams, which travel in different directions, and are polarized perpendicular to each other.

Linear polarization can be demonstrated with a set of two polarizers. If a ray of light passes the first polarizer, it will be converted to linearly polarized light. When the linearly polarized light is incident on a second polarizer, commonly referred to as an analyzer, with its polarization axis parallel to that of the first polarizer, the light emerging from the second polarizer will still be linearly polarized. If the analyzer is rotated by 90 degrees, it will completely attenuate the light.

Another polarization state of light is referred to as circular polarization. In circularly polarized light, the electric vector follows a helical (spiral) path around the propagation direction, as shown in figure 1d. The sense of the helical motion can be either right- or left-handed; thus this polarization of light is referred to as right or left circularly polarized light. The observer at point O would perceive the electric field (the electric vector) to rotate clockwise or counterclockwise around the propagation direction, as shown in figure 1e.

The creation of circularly polarized light can be visualized from figure 1d. Circular polarization is achieved when two colinear light rays, which are linearly polarized at right angles to each other, interfere, and when one of the waves lags behind or leads by exactly one quarter of a wavelength.

In any wave interference, the resulting amplitude of the electric field is the sum of the individual amplitudes; thus, the instantaneous values of the electric fields of each ray need to be added. The resulting electric field describes a helical, or spiral, path about the propagation direction. The spiral will be left- or right-handed.

The exact interference of the two rays is achieved, in practice, by dividing an incoming electromagnetic beam into two components and retarding one component over the other. This effect can be accomplished using birefringent optical crystals, if they are oriented such that the ordinary and extraordinary refracted rays are coincident. Since these two rays are always polarized perpendicular to each other, it is necessary to adjust the thickness of the birefringent material so that upon leaving it, the two waves have experienced a phase shift of λ/4.

Thus, birefringent plates used to produce circularly polarized light are also known as λ/4 plates. Common minerals that have the property of producing circularly polarized light are mica and quartz.

In addition to the polarization states, a number of other polarizations exist that are a superposition of random, linear, and circular polarizations.

Applications

The principles of polarization are utilized in a wide variety of commercial and scientific fields. Polarizing sunglasses, for example, use Polaroid sheets to reduce the intensity of light transmitted through the glasses. Since the light reflected from surfaces, such as a roadway or water, is partially polarized, polarizing sunglasses will cut out the glare of such reflections much better than sunglasses that merely utilize gray glass to reduce light intensities.

The principle of polarization by reflection has widespread applications. For example, the output beams of most gas lasers are linearly polarized, since the windows that terminate the laser tubes are attached at Brewster's angle. The reason for using Brewster's angle on these windows is twofold: First, linear polarization of the laser beam is achieved, which is advantageous for many applications. Second, since the refracted beam is at maximum intensity at Brewster's angle, the reflection losses at the windows are minimized.

Polarizers based on birefringent materials are used extensively in laser optics application, where polarizers are required that can withstand the high-power density laser beams.

These prism polarizers utilize two prism-shaped pieces of quartz or calcite, cemented together so that either the ordinary or the extraordinary beam is removed at the interface between the prisms.

Thus, linearly polarized light is transmitted.

Birefringent materials are used extensively in optics to produce different polarization properties of light. Plates of λ/4 are used to produce circularly polarized light, and plates of twice the thickness, which induce a λ/2 phase shift, are used to rotate the polarization of light by 90 degrees. Thus, these plates are known as polarization rotators.

Many materials that are normally not birefringent can be rendered artificially birefringent when they are subjected to mechanical or electrical stress. "Stress birefringence" and "electric birefringence" have gained importance in many fields of modern optical and engineering research. In mechanical engineering, for example, stress birefringence can be applied to study the behavior of structural elements. Models of such structural elements are constructed from transparent materials, subjected to expected loads, which cause stress, and examined with polarized light. At points of stress in these structural elements, stress birefringence changes the polarization of the transmitted light, which will appear as a dark area when the transmitted light is analyzed through a second polarizer. Stress patterns and their dependence on the shape of the structural elements can be studied.

Electrical birefringence, or the electro-optic effect, is used in many aspects of modern technology. The creation of very short laser pulses, for example, can be achieved by having an electro-optic crystal in a laser beam. When an appropriate voltage is applied to such a crystal, it will act as a λ/2 plate, that is, it will rotate the polarization plane of a laser by 90 degrees, such that it cannot pass a polarizing element. Reducing the voltage momentarily to zero, the polarization plane is not rotated, and the laser beam can pass the polarizer. Thus, light pulses can be produced, which are synchronized to electrical signals. Such techniques are employed in fiber-optic communications. Electro-optic switches can also be produced, which can direct light pulses into different directions, just as a switch on a railroad track can send trains onto different tracks.

The change in polarization upon reflection is utilized in an area known as ellipsometry.

In ellipsometry, thin films deposited on reflecting surfaces will alter the polarization of the reflected light differently than the clean surface, and information on the nature and structure of the molecules in the deposited film can be obtained.

The most important applications of polarization effects are found in spectroscopy. All forms of spectroscopy study the interaction of light with molecules to identify the molecules or deduce physical and chemical properties of these molecules. Here, polarization effects are of prime importance and can reveal the orientation of parts of molecules with respect to one another, the orientation of molecules in a crystal, the shapes and symmetry of molecules, and many other properties.

Circularly polarized light interacts with some matter differently than linearly or randomly polarized light. Certain molecules that form helical structures (proteins and nucleic acids) even have the property of distinguishing between left and right circularly polarized light.

This property, known as circular dichroism, has been used extensively in the study of the shape of biomolecules, which assume helical structures.

Context

The polarization of light, and the changes in the polarization state when light undergoes various optical effects such as reflection, refraction, or scattering, are well understood, and are, per se, no longer the subjects of modern studies in physics. Nevertheless, polarization effects can shed light on many problems in engineering, chemistry, and physics. Thus, the study of light polarization has shifted from the phenomenological concepts to applications.

The concept of the polarization of light was developed in the seventeenth century, when detailed and quantitative experiments in optics were carried out. Christiaan Huygens, a Dutch astronomer and physicist, formulated the principles of the wave nature of light and discovered the polarization of light by double refraction in calcite.

Mathematical models describing the wave and polarization properties of light were fully developed during the eighteenth and nineteenth centuries. During this period, optics was already at a very sophisticated stage, much more so than other branches of science. Many effects were described quantitatively, such as polarization by reflection, which was put in mathematical form by Brewster in 1812.

Quantum theory, developed early in the twentieth century, helped to formulate the present-day picture of polarization and the wave-particle duality of light. The advent of lasers and laser optics has brought new interest to the field of polarization optics and since then, many applications of polarized light have emerged.

Principal terms

ELECTRIC FIELD: one component of electromagnetic radiation; a static electric field is a change in space, caused by an electric charge, such that another electric charge experiences a force

ELECTROMAGNETIC RADIATION: a general term for visible and invisible forms of light; composed of oscillating electric and magnetic fields

LINEAR POLARIZATION: light in which the electric field oscillates in one plane

MAGNETIC FIELD: a component of electromagnetic radiation; a change in space caused by a moving charge, such that a magnetic dipole (such as a compass) experiences a force

POLARIZATION: the direction of oscillation of the electric (or magnetic) field in reference to the propagation direction of the light beam

POLARIZER: a device that can produce linearly polarized light

Bibliography

Halliday, David, and Robert Resnick. PHYSICS: PART II. New York: John Wiley & Sons, 1960. General college-level physics text, with good treatment of classical optics.

Hecht, Eugene. OPTICS. Reading, Mass.: Addison-Wesley, 1987. Offers an excellent introduction to polarization effects, circular polarization, interference of waves, and other areas. For the nontechnical reader.

Jenkins, Francis A., and Harvey E. White. FUNDAMENTALS OF OPTICS. New York: McGraw-Hill, 1957. Offers an excellent treatment of general optics and polarization effects. Chapters 24, 26, and 27 contain relevant information.

Nussbaum, Allen. CONTEMPORARY OPTICS FOR SCIENTISTS AND ENGINEERS. Englewood Cliffs, N.J.: Prentice-Hall, 1976. Gives a solid introduction to the optical principles necessary for the understanding of polarization of light.

Sears, Francis W. OPTICS. Reading, Mass.: Addison-Wesley, 1958. An excellent text written primarily for college-level physics courses. Useful illustrations in the chapter on polarization make it particularly readable, even for a novice in the field.

Reflection and Refraction