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Waves come in many different forms. We generally distinguish between two kinds of waves—longitudinal waves (for example, sound waves) and transverse waves (for example, electromagnetic waves such as light or radio waves). The difference between the two types lies in the manner by which the wave propagates. Longitudinal waves advance in the direction of the disturbance they create. For instance, sound waves originate in the motion of air molecules. The movement of one group of molecules sets additional molecules in motion, and in this way the sound wave travels. In other words, the wave moves together with the spreading disturbance, which in this case is the motion of the molecules. By contrast, in transverse waves the disturbance propagates perpendicular to the direction of propagation.

Wave polarization describes the direction of oscillations as a function of time. Nevertheless, a wave need not have a well-defined polarization, because the oscillation direction can change as a function of time and not necessarily in a predictable way.

Typically, when referring to polarization we mean the polarization of light waves, which are electromagnetic waves. As its name implies, an electromagnetic wave is actually composed of two fields—electric and magnetic—that are perpendicular to each other. The polarization direction is defined as the direction of the electric field oscillations. In electromagnetic waves in vacuum, both the electric field and the magnetic field are perpendicular to the direction of wave propagation.

There are several polarization states:
Unpolarized light—meaning there is no fixed direction for the electric field oscillations. The best example of unpolarized light is sunlight. A second state is linearly polarized light, in which the electric field oscillates in a single plane whose direction does not change with time. A third state is circular polarized light, in which the electric field changes its direction over time in a circular fashion (while keeping a constant amplitude). This state can appear in two forms: right-hand circular polarization, when the field rotates clockwise, and left-hand circular polarization, when the field rotates counter-clockwise. The last state is called elliptical polarization, describing a scenario in which the electric field changes both its direction and its amplitude over time.

Up to this point we have explained this property of light, which at first glance seems of little practical use, mainly because it is not intuitive. Yet polarization has countless applications in industry and in nature. It is possible to create a component that polarizes incident light into a desired state. For example, unpolarized light passing through a linear polarizer (a component whose output is linearly polarized light) loses about half of its intensity, in accordance with Malus’ law. This property is extremely useful for producing sunglasses with polarized lenses which, unlike regular sunglasses, change only the brightness of the view rather than its color.

Today polarizers are also used in glasses for viewing 3D broadcasts. Some of you may remember that in the past glasses with one blue filter and one red filter were used, and the image was projected twice with a certain offset to create a 3D effect. In the more advanced technology, sunglasses with linear polarizers are used, where the polarizer in the right eye is rotated by 90° relative to the one in the left eye. The image is again projected twice with a certain offset, but with polarizations matched to the glasses, so that each eye receives only the projection intended for it, thereby creating the illusion of depth.

Many animals can detect the polarization of light and use it for navigation (among other purposes), because light in the sky at angles far from the Sun is linearly polarized and its polarization direction is always perpendicular to the Sun.

Certain materials under mechanical stress affect the polarization of light incident on them, and by measuring the change one can determine the stress applied to the material [8]. Additional applications can be found in astronomy, geology, chemistry, and more.

In principle, humans are also capable of perceiving light polarization via an optical phenomenon called Haidinger’s brush, and it is even possible to train the eye to detect it more easily. If you look at the sky at an angle far from the Sun, so that the light reaching you is polarized, with only modest effort you can discern a yellow butterfly-shaped pattern against the sky. You can perform the same experiment at home: look through polarized sunglasses at a brightly lit white wall.

English editing: Elee Shimshoni