Polarization (waves)

In physics, polarization refers to the orientation of the direction in which a wave is oscillating as it travels. The process is most often associated with electromagnetic waves such as light waves. These types of waves typically vibrate in more than one direction as they move. Polarization can filter out or limit that vibration to only one direction. The term comes from the French polarizer, which means to “accentuate differences,” and was originally based on the Greek word, polos, or the “axis of a globe.” The process of polarization is used to make sunglasses that reduce glare and in motion picture technology to make 3-D films.

Background

Light is a form of energy that is also known as electromagnetic radiation. Light is produced when an atom absorbs energy, creating an electric field that accelerates subatomic particles known as electrons. As the electrons move, they produce oscillating electric and magnetic fields that travel as a particle of light energy called a photon. Photons move incredibly fast at about 186,000 miles per second (299,300 kilometers per second)—a figure referred to as the speed of light.

The oscillating electric and magnetic fields move at right angles to each other and travel in the form of waves. Light waves have peaks and valleys that determine the light’s frequency and wavelength. The measure of a wave from peak to peak is known as the wavelength, while frequency refers to the number of wave peaks that pass a point in a certain period of time. The visible light humans can see falls within a narrow band of frequencies and wavelengths. Longer wavelengths have lower frequencies because fewer wave peaks pass by per second. Radio waves, microwaves, and infrared light are examples of this low-frequency electromagnetic radiation. Waves with short wavelengths have high frequencies and contain more energy than those with long wavelengths. Examples of these types of electromagnetic radiation include ultraviolet light, X-rays, and gamma rays.

Light is a transverse wave. In this type of wave, the elements of the wave—in the case of light, the electric and magnetic fields—oscillate, or vibrate, perpendicular to the overall direction the wave is traveling in. For example, if a wave is traveling in a right-left direction, the electric field would be vibrating up and down at a 90-degree angle in relation to the wave movement. The magnetic field would be vibrating side to side at 90 degrees in relation to the movement. The electromagnetic fields are not locked into these specific orientations and can oscillate in any direction as long as it is perpendicular to the wave movement.

Overview

The process of polarization mainly concerns the direction of the electric field as the light wave travels. Most light is unpolarized, which means that the electric field is oscillating in multiple directions as the wave moves. The light from the sun and the light from a light bulb are both common examples of unpolarized light. In polarized light, the electric field vibrates in just one direction. It could be moving up and down, side to side, or diagonally, but it moves in only one plane, and its overall orientation remains perpendicular to the wave motion.

The idea of polarized light was first hinted at in experiments conducted by seventeenth-century Danish scientist Erasmus Bartholin, who noticed how light passing through a mineral crystal produced a series of images and that one image seemed to move around the others as the crystal was rotated. This suggested to Bartholin that the crystal somehow split the light into two separate beams. In the eighteenth century, French physicist Etienne Malus found that when sunlight passed through a crystal, it produced multiple images. He then discovered that he could manipulate the experiment to make one of the images disappear. From this, he theorized that sunlight consisted of two different kinds of light; later scientists determined that the effect was caused by different polarizations of sunlight.

The process of transforming unpolarized light into polarized light is called polarization. This can be accomplished in several different ways. A common polarization effect can be observed in light reflecting off a nonmetallic surface. Metallic surfaces reflect light on a number of different oscillation planes, so the light remains unpolarized. Polarizing light by reflecting it depends on the material the reflecting substance is made of and the angle at which the light hits the surface. For example, a field of freshly fallen snow or the waters of a lake can reflect light at an angle parallel to the surface, which concentrates the oscillations of the electric field into a single plane. If the polarization effect is large enough, a person may perceive the light as a glare reflected off the surface. If light is reflected at a very specific angle, all the light waves will line up and be perfectly polarized. This phenomenon is called Brewster’s angle, named after nineteenth-century Scottish physicist David Brewster.

Malus made his observations by viewing refracted light. Refraction occurs when light passes from one medium through another, such as when light passes from the air through a glass lens. The path of the light beam changes direction when it encounters the surface of a new medium. Typically, refracted light is polarized at an angle perpendicular to the surface but can also occur at other angles as well. In some cases, a surface can refract the light into two beams, each polarized in a different plane. By using a filter to remove one of the polarized beams, one of the two images produced by the light will vanish. Adjusting the filter can bring the image back and make the other vanish.

Light can also be polarized through the effects of being scattered by a medium. When light passes through a substance such as air, it often strikes the molecules within the substance. This transfer of energy causes the electrons in the molecules to produce their own oscillating electromagnetic waves, scattering and polarizing the light. This effect is what makes the sky blue, as the blue frequencies of light are scattered by Earth’s atmosphere. Similar to reflected light, light polarized by scattering can produce glare.

The easiest and most practical way to polarize light is through the use of a polarizing filter. The first polarizing filters were invented in the early nineteenth century and were basically just stacks of glass sheets pressed together. Modern filters are made of polymer materials that consist of long chains of molecules. These molecules are stretched and aligned in a single direction meant to block light oscillating in a specific orientation. Light oscillating parallel, or in the same plane as the polymer molecules, is absorbed, while light oriented perpendicular, or at a 90-degree angle to that plane, passes through. The light emerges from the filter polarized but also at a lesser intensity than the original light source. In many cases, the filter can be realigned to absorb different orientations.

Polarizing filters are commonly used in photography to darken the image, lessen the effects of reflections, or reduce glare. The same material is also incorporated into eyewear to make sunglasses that cut down on glare. The sunglasses are meant to absorb light that is oscillating in a plane parallel to a surface; light oscillating perpendicular to the surface will pass through. Polarizing filters are also used by manufacturers to test the durability of transparent plastics. As light is passed through a plastic, different polarizations can produce different colors. A high concentration of colors likely indicates an area of structural concern.

Perhaps the most familiar use of polarization involves the process of making and showing 3-D films on theater movie screens. A 3-D film is really two movies being projected onto the screen at the same time. In many cases, filmmakers use special cameras to record two shots of every image, or films can be converted through a special process after they have been completed. When the movie is shown on a theater screen, it is projected through a polarizing filter from two different locations on either side of an audience. The filter on the right may be aligned to absorb horizontal polarization; the one on the left may be set to absorb vertical polarization. To get the 3-D effect, the audience wears special glasses with a different polarizing filter in each lens. The lens on the right has an alignment that allows light to enter from the projector on the left; the left lens allows in light from the opposite projector. This arrangement gives the audience the illusion of depth on a two-dimensional screen.

Bibliography

Barras, Colin. “What Is a Ray of Light Made of?” BBC, 31 July 2015, www.bbc.com/earth/story/20150731-what-is-a-ray-of-light-made-of. Accessed 27 June 2019.

“Chapter 4 Polarization.” University of Michigan, instructor.physics.lsa.umich.edu/int-labs/Chapter4.pdf. Accessed 27 June 2019.

Elert, Glenn. “Polarization.” Physics Hypertextbook, 2019, physics.info/polarization/. Accessed 27 June 2019.

Feltman, Rachel. “Seeing Double: How Do 3D Movies Really Work?” Scienceline, 13 Dec. 2012, scienceline.org/2012/12/seeing-double-how-do-3d-movies-really-work/. Accessed 27 June 2019.

Lui, Wen, et al. "Arbitrarily Rotating Polarization Direction and Manipulating Phases in Linear and Nonlinear Ways Using Programmable Metasurface." Light: Science & Applications, vol. 13, no. 172, 18 July 2024, doi.org/10.1038/s41377-024-01513-2. Accessed 17 Nov. 2024.

Lucas, Jim. “What Is Electromagnetic Radiation?” Live Science, 12 Mar. 2015, www.livescience.com/38169-electromagnetism.html. Accessed 27 June 2019.

Murphy, Douglas B., Kenneth R. Spring, and Michael W. Davidson. “Introduction to Polarized Light.” Microscopy U, 2019, www.microscopyu.com/techniques/polarized-light/introduction-to-polarized-light. Accessed 27 June 2019.

“Polarization.” The Physics Classroom, 2019, www.physicsclassroom.com/class/light/Lesson-1/Polarization. Accessed 27 June 2019.

Roychoudhuri, Chandra, A.F. Kracklauer, and Kathy Creath, editors. The Nature of Light: What is a Photon? CRC Press, 2008.