What is polarization of light?

Light is an electromagnetic wave consisting of oscillating electrical and magnetic fields in directions perpendicular to one another and has many properties, from acting with particle behavior (photons or quanta of energy) to exhibiting wave-like behaviors. Another property of light is polarization, a notably harnessed feature in sunglass technology to reduce the intensity of light reaching the observer’s eye or dim the glare from any bright surface like water or the sky. But what, concretely, explains the polarization of light, reducing its intensity?

As stated, light vibrates and propagates in several planes and can thus be seen by all observers from all directions: light is then referred to as unpolarized. However, when light passes through a polarizing filter, only the rays of light that oscillate in a said plane are let through the filter. A polarizing filter or polarizer works by filtering the direction of oscillations of a beam of light with undefined polarization into a beam of light with well-defined polarization.

Polarized light
Image: Encyclopædia Britannica, Inc.

Polarized light can thus be either studied or used when light is intentionally polarized. But when polarized light occurs naturally, scientists may want to study the cause behind that polarization to understand better the environment from where the beam of light is reflected or radiated. Indeed, many of these applications are found in various fields of astronomy, allowing us to study the atmosphere of planets or interstellar magnetic fields.

Types of polarization

Observing The Universe With Polarized Light Reveals A Lot More Than We Normally See. 2
Image: HyperPhysics

We generally classify polarization into two main categories, circular and linear polarization (there’s also a third type called elliptical polarization, but for the sake of simplicity, we’ll stick to circular and linear). Linear polarization can be generated in two different ways. Firstly, thanks to absorptive polarizers, the beam could be polarized by absorbing all the other unwanted planes of oscillation in the polarizer, thus only letting a single plane of oscillation in the beam of light through. Common materials used as absorptive polarizers are tourmaline, a crystal, polyvinyl alcohol sheets (or plastic Polaroid film), and elongated silver nano-particles mostly used in fiber optics communications as they work best for short-wavelength light.

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On the other hand, the light may also be linearly polarized through beam-splitting polarizers, which split the incoming beam into two subsequent ones of complementary polarization and are more suitable for the analysis of high-intensity light such as lasers.

Secondly, circular polarizing filters generate polarized light with different directions of rotations, that is, either clockwise or counter-clockwise circularly polarized light. This type of polarized light is most commonly used in photography to reduce any oblique reflection and receive a beam of light free from glare. Also, these filters are used in 3D glasses, for example, to differentiate between the images sent to be perceived by the left eye and those intended for the right eye.

Polarization in astronomy

When studying the properties of the Sun, astronomers have observed both linear and circular polarization. Just like in other stars, this can reveal information about the areas and intensity of stellar magnetic fields. Indeed, from the polarized light emitted from the Sun and other stars, astronomers were able to develop a secondary solar or stellar spectrum that does not plot the visible light of the star but rather the solar spectrum which shows the polarized light of the sun. Indeed, across the limb or surface of the Sun, light is polarized more or less at different areas.

Polarization in astronomy
A visualization of the visible part of the solar spectrum (left) and second solar spectrum at the solar limb (right).

However, these individual areas of polarization cancel out overall, so the sun’s secondary spectrum is a valuable tool in understanding individual areas of polarization and their relative intensities. The observed spectrum may reveal areas of weaker magnetic fields as atoms may be polarized due to them. Therefore, polarized light and its spectrum help astronomers understand and map out stellar magnetic fields.

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