Polarization of Light

Polarization of Light

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Understanding and manipulating the polarization of light is crucial for many optical applications. Optical design frequently focuses on the wavelength and intensity of light, while neglecting its polarization. Polarization, however, is an important property of light that affects even those optical systems that do not explicitly measure it. 

The polarization of light affects the focus of laser beams, influences the cut-off wavelengths of filters, and can be important to prevent unwanted back reflections. It is essential for many metrology applications such as stress analysis in glass or plastic, pharmaceutical ingredient analysis, and biological microscopy. Different polarizations of light can also be absorbed to different degrees by materials, an essential property for LCD screens, 3D movies, and glare-reducing sunglasses.

What is Polarization of Light?

Polarization, property of certain electromagnetic radiations in which the direction and magnitude of the vibrating electric field are related in a specified way.

Light waves are transverse: that is, the vibrating electric vector associated with each wave is perpendicular to the direction of propagation. A beam of unpolarized light consists of waves moving in the same direction with their electric vectors pointing in random orientations about the axis of propagation. Plane polarized light consists of waves in which the direction of vibration is the same for all waves. 

In circular polarization the electric vector rotates about the direction of propagation as the wave progresses. Light may be polarized by reflection or by passing it through filters, such as certain crystals, that transmit vibration in one plane but not in others.

Types of Polarization

Depending on how the electric field is oriented, we classify polarized light into three types of polarizations:

  • Linear Polarization: the electric field of light is confined to a single plane along the direction of propagation. 
  • Circular Polarization: the electric field of the light consists of two linear components that are perpendicular to each other, equal in amplitude, but have a phase difference of π/2. The resulting electric field rotates in a circle around the direction of propagation and, depending on the rotation direction, is called left- or right-hand circularly polarized light. 
  • Elliptical Polarization: the electric field of light describes an ellipse. This results from the combination of two linear components with different amplitudes and/or a phase difference that is not π/2. This is the most general description of polarized light, and circular and linear polarized light can be viewed as special cases of elliptically polarized light.  

The two orthogonal linear polarization states that are most important for reflection and transmission are referred to as p- and s-polarization. p-polarized (from the German parallel) light has an electric field polarized parallel to the plane of incidence, while s-polarized (from the German senkrecht) light is perpendicular to this plane.

Terms associated with Polarization of Light

Following are the commonly used terms associated with polarization of light:

  • Plane of Vibration: The plane in which vibration of electric field vectors are restricted are termed as plane of vibration.
Polarization of Light
Left: Vertical, Right: Horizontal Polarization of Light

The figure represents the motion of plane polarized light in both vertical and horizontal directions respectively.

  • Plane of Polarization: The plane which is perpendicular to the plane of vibration i.e., the plane in which there is no vibration of electric field vectors are known as planes of polarization.
Polarization of Light
  • Polarizers: The device which is used to polarize an unpolarized light are known as polarizers and some of its examples are Nicol prism, Tourmaline crystal, etc.
  • Analyzer: The device which is used to determine the plane of polarization is termed as analyzer.
  • Polaroids: It is a large sheet made up of microscopic dichroic crystals which can produce a beam of polarized light.

Applications of Polarization of Light

Following are some of the common applications of polarization of light:

Sunglasses

The polarization of light can be observed easily by looking at the structure of sunglasses or goggles. Here, the polarization property of light radiations is used to reduce glare and provide comfort to the eyes of the user.

Spectroscopy

A number of spectroscopy techniques such as infrared spectroscopy make use of the polarization of light.

Three-Dimensional Movies

The making of three-dimensional movies or pictures is typically possible due to the polarization property of radiation. A three-dimensional movie typically consists of two separate rolls of film. These two film rolls are used to record the same scenario but from different angles. The projector that is used to display the three-dimensional movie typically consists of two different lenses that are connected to their individual polarizers. 

Both the polarizers are located at right angles to each other. The lenses of the three-dimensional glasses that are used to see the three-dimensional movies are cross-polarized. This means that the left side of the three-dimensional glass corresponds to the left lens of the camera, while the right side of the three-dimensional glass is analogous to the right lens of the recording camera. This helps display the movie in the exact format as it was recorded.

Industries

Various industries such as plastic industries, chemical industries, metallurgy and smithy factories, etc. make use of polarization of radiations to carry out tests for stress and pressure analysis of objects. One of the most common testing procedures used by such industries includes optical stress analysis. 

Optical stress analysis makes use of plastic models to determine the regions of potential weaknesses of the material. When the material is put under stress, a stress pattern is formed that contains various bands of light and dark colour. The image of the stress pattern is generally formed with the help of polarizers.

Seismology

Seismology or the study of the nature of earthquakes make use of polarization of radiations to determine the magnitude of an earthquake. It also helps scientists and researchers to make an attempt and predict the occurrence of a similar earthquake in the near future.

Chemistry Laboratories

A number of testing and experiment procedures that take place at chemistry laboratories make use of the polarization of radiations. For instance, the process of testing the chirality of the organic compounds mainly works on the principle of polarization.

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