WHAT IS POLARIZED LIGHT? - PHYSICAL - 2024

The polarized light is electromagnetic radiation vibrating in one plane perpendicular to the propagation direction. The vibration in a plane means that the electric field vector of the light wave oscillates parallel to a space of two rectangular components, as is the case of the xy plane of polarization.

Natural or artificial light is a wave train of electromagnetic radiation whose electric fields oscillate randomly in all planes perpendicular to the direction of propagation. When only a portion of the radiation is restricted to oscillating in a single plane, the light is said to be polarized.

Vertically polarized light wave in a plane as non-polarized light waves impinge on a polarization grating. By Bob Melish (https://commons.wikimedia.org/wiki/File:Wire-grid-polarizer.svg) Wikimedia Commons

One way to obtain polarized light is by striking a ray of light on a polarizing filter, which consists of a polymer structure oriented in a single direction, allowing only waves that oscillate in the same plane to pass through while the rest of the waves are absorbed..

The ray of light that passes through the filter has a lower intensity than the incident ray. This feature is a way to distinguish between polarized light and non-polarized light. The human eye does not have the ability to distinguish between one and the other.

Light can be linear, circular or elliptical polarized depending on the direction of the wave propagation. Also, polarized light can be obtained by physical processes such as reflection, refraction, diffraction, and birefringence.

Linearly polarized light

When the electric field of the light wave oscillates constantly, describing a straight line in the plane perpendicular to the propagation, the light is said to be linearly polarized. In this state of polarization the phases of the two components of the electric field are the same.

If two linearly polarized waves, vibrating in planes perpendicular to each other, are superimposed, another linearly polarized wave is obtained. The light wave obtained will be in phase with the previous ones. Two waves are in phase when they present the same displacement in the same time.

Linear, circular and elliptical polarization. By Inductiveload. (https://commons.wikimedia.org)

Circular polarized light

The light wave whose electric field vector oscillates in a circular way in the same plane perpendicular to the propagation, is circularly polarized. In this state of polarization the magnitude of the electric field remains constant. The orientation of the electric field is clockwise or counterclockwise.

The electric field of polarized light describes circular paths with a constant angular frequency ω.

Two linearly polarized light waves that are superimposed perpendicular to each other, with a phase difference of 90 °, form a circularly polarized light wave.

Elliptically polarized light

In this state of polarization, the electric field of the light wave describes an ellipse in the entire plane perpendicular to the propagation and is oriented in a clockwise or counterclockwise direction of rotation.

The superposition of two light waves perpendicular to each other, one with linear polarization and the other with circular polarization, and with a phase shift of 90 °, results in a light wave with elliptical polarization. The polarized light wave is similar to the case of circular polarization but with the magnitude of the electric field varying.

Reflection polarized light

Reflection-polarized light was discovered by Malus in 1808. Malus observed that when a beam of nonpolarized light strikes a well-polished, transparent glass plate, part of the light is refracted as it passes through the plate and the other part is reflected, forming an angle of 90 ° between the refracted ray and the reflected ray.

The reflected light beam is linearly polarized when oscillating in a plane perpendicular to the direction of propagation and its degree of polarization depends on the angle of incidence.

The angle of incidence by which the reflected light beam is fully polarized is called the Brewster angle (θ _B)

Refraction polarized light

If an unpolarized beam of light is incident with a Brewster angle (θ _B) on a stack of glass plates, some of the vibrations perpendicular to the plane of incidence are reflected from each of the plates and the rest of the vibrations are refracted.

The net result is that all the reflected beams are polarized in the same plane while the refracted beams are partially polarized.

The greater the number of surfaces, the refracted ray will lose more and more oscillations perpendicular to the plane. Ultimately the transmitted light will be linearly polarized in the same plane of incidence as the nonpolarized light.

Scattering polarized light

The light that falls on small particles suspended in a medium is absorbed by its atomic structure. The electric field induced in the atoms and molecules has vibrations parallel to the plane of oscillation of the incident light.

Likewise, the electric field is perpendicular to the direction of propagation. During this process the atoms emit photons of light that are deflected in all possible directions.

The emitted photons constitute a set of waves of light scattered by the particles. The portion of the scattered light perpendicular to the incident light beam is linearly polarized. The other portion of light scattered in the parallel direction is not polarized, the rest of the light scattered by the particles is partially polarized.

The scattering of particles with a size comparable to the wavelength of the incident light is called Rayleigh scattering. This type of scattering makes it possible to explain the blue color of the sky or the red color of the sunset.

Rayleigh scattering has a dependence inversely proportional to the fourth power of the wavelength (1 / λ ⁴).

Birefringence polarized light

Birefringence is a characteristic property of some materials such as calcite and quartz that have two refractive indices. Birefringent polarized light is obtained when a ray of light falls on a birefringent material, separating into a reflected ray and two refracted rays.

Of the two refracted rays, one deviates more than the other, oscillating perpendicular to the plane of incidence, while the other oscillates parallel. Both rays emerge from the material with linear polarization to the plane of incidence.

References

1. Goldstein, D. Polarized Light. New York: Marcel Dekker, inc, 2003.
2. Jenkins, FA and White, H E. Fundamentals of Optics. NY: McGraw Hill Higher Education, 2001.
3. Saleh, Bahaa E. A and Teich, M C. Fundamentals of photonics. Canada: John Wiley & Sons, 1991.
4. Guenther, R D. Modern Optics. Canada: John Wiley & Sons, 1990.
5. Bohren, CF and Huffman, D R. Absorption and scattering of light by small particles. Canada: Jhon Wiley & Sons, 1998.