Natural or artificial light is a train of waves of electromagnetic radiation whose electric fields oscillate randomly in all planes perpendicular to the direction of propagation. When only a part of the radiation is restricted to oscillating in a single plane, the light is said to be polarized.
One way to get polarized light is to have a beam of light affect a polarizing filter, which consists of a polymer structure oriented in only one direction, allowing only waves that oscillate in the same plane to pass through while the rest of the waves are absorbed. .
The beam of light that passes through the filter has less intensity than the incident beam. This feature is a way of distinguishing 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 polarized linearly, circularly or elliptically depending on the direction of wave propagation. Furthermore, polarized light can be obtained by physical processes such as reflection, refraction, diffraction and birefringence.
linear polarized light
When the electric field of the light wave constantly oscillates, 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 are superimposed, which vibrate in planes perpendicular to each other, 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 have the same displacement at the same time.
circular polarized light
A light wave whose electric field vector oscillates circularly 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 either clockwise or counterclockwise.
The electric field of polarized light describes circular paths with a constant angular frequency ω .
Two linearly polarized light waves that overlap perpendicularly 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 one with circular polarization and with a displacement of 90°, results in a light wave with elliptical polarization. The polarized light wave is similar to the circular polarization case, but with the magnitude of the electric field varying.
reflection polarized light
Reflected polarized light was discovered by Malus in 1808. Malus observed that when a beam of unpolarized light hits a well-polished, transparent glass plate, some of the light is refracted as it passes through the plate and the other part is reflected to form A 90° angle between the refracted ray and the reflected ray.
The reflected light beam polarizes linearly when it oscillates in a plane perpendicular to the propagation direction and its degree of polarization depends on the angle of incidence.
The angle of incidence through which the reflected light beam is fully polarized is called the Brewster angle (θ B )
Refraction polarized light
If a beam of unpolarized light reaches the Brewster angle ( θ B ) in a stack of glass plates, some of the vibrations perpendicular to the plane of incidence are reflected in each of the plates and the rest of the vibrations are refracted.
The end result is that all reflected rays are polarized in the same plane, while the refracted rays are partially polarized.
The greater the number of surfaces, the refracted ray will lose more and more oscillations perpendicular to the plane. In the end, transmitted light will be linearly polarized in the same plane of incidence as unpolarized light.
Polarized Light Scattering
Light affecting small particles suspended in a medium is absorbed by their atomic structure. The electric field induced in atoms and molecules has vibrations parallel to the plane of oscillation of the incident light.
Likewise, the electric field is perpendicular to the propagation direction. During this process, atoms emit photons of light that bend in all possible directions.
The emitted photons constitute a set of light waves scattered by the particles. The portion of scattered light perpendicular to the incident light beam is linearly polarized. The other portion of light scattered in a 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 incident light is called Rayleigh scattering. This type of dispersion explains 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, which have two refractive indices. Birefringence polarized light is obtained when a beam of light strikes a birefringent material that separates 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 in parallel. Both rays emerge from the material with linear polarization towards the plane of incidence.