The light that most light sources emit, such as the Sun or a light bulb, is non-polarized, which means that both components: electrical and magnetic, oscillate in all possible directions, although always perpendicular to the direction of propagation.
But when there is a preferential or unique direction of oscillation of the electrical component then we speak of a polarized electromagnetic wave. Furthermore, if the frequency of the oscillation is in the visible spectrum, then we speak of polarized light.
Next we will look at the types of polarization and the physical phenomena that produce polarized light.
Types of polarization
Linear polarization occurs when the plane of oscillation of the electric field of the light wave has a single direction, perpendicular to the direction of propagation. This plane is taken, by convention, as the plane of polarization.
And the magnetic component behaves the same: its direction is perpendicular to the electrical component of the wave, it is unique and it is also perpendicular to the direction of propagation.
The upper figure shows a linearly polarized wave. In the case shown, the electric field vector oscillates parallel to the X axis, while the magnetic field vector oscillates simultaneously to the electric one, but in the Y direction. Both oscillations are perpendicular to the Z direction of propagation.
Oblique linear polarization can be had as a result of the superposition of two waves that oscillate in phase and have orthogonal polarization planes, as the case shown in the figure below, where the oscillation plane of the electric field in the light wave is shown in blue.
In this case, the amplitude of the electric and magnetic fields of the light wave has constant magnitude, but its direction rotates with constant angular speed in the direction transverse to the direction of propagation.
The lower figure shows the rotation of the amplitude of the electric field (in red color). This rotation results from the sum or superposition of two waves with the same amplitude and linearly polarized in orthogonal planes, whose phase difference is π / 2 radians. They are represented in the figure below as blue and green waves respectively.
The way to mathematically write the x and y components of the electric field of a wave with right-hand circular polarization , of amplitude Eo and propagating in the z direction is:
E = (Ex i ; Ey j ; Ez k ) = Eo (Cos [(2π / λ) (ct – z)] i ; Cos [(2π / λ) (ct – z) – π / 2] j ; 0 k )
On the other hand, a wave with left-hand circular polarization of amplitude Eo propagating in the z direction is represented by:
E = (Ex i ; Ey j ; Ez k ) = Eo (Cos [(2π / λ) (ct – z)] i , Cos [(2π / λ) (ct – z) + π / 2] j , 0 k )
Note that the sign is changed by the phase difference of a quarter wave of the y component , with respect to the x component.
Both for dextrorotatory and levorotatory , the vector magnetic field B is related to the electric field vector E using vector product between the unit vector in the direction of propagation and E , including a scaling factor equal to the inverse of the speed of light :
B = (1 / c) û zｘ E
Elliptical polarization is similar to circular polarization, with the difference that the amplitude of the field rotates describing an ellipse instead of a circle.
The wave with elliptical polarization is the superposition of two linearly polarized waves in perpendicular planes with an advance or delay of π / 2 radians in the phase of one with respect to the other, but with the addition that the amplitude of the field in each of the components is different.
Phenomena due to light polarization
When a non-polarized light beam strikes a surface, for example glass, or the surface of water, part of the light is reflected and part is transmitted. The reflected component is partially polarized, unless the incidence of the beam is perpendicular to the surface.
In the particular case that the angle of the reflected beam forms a right angle with the transmitted beam, the reflected light has total linear polarization, in the direction normal to the plane of incidence and parallel to the reflecting surface. The angle of incidence that produces full polarization by reflection is known as the Brewster angle.
Some materials allow the selective transmission of a certain plane of polarization of the electrical component of the light wave.
This is the property that is used for the manufacture of polarizing filters, in which an iodine-based polymer is generally used, stretched to the limit and aligned as a grid, compacted between two sheets of glass.
Such an arrangement acts as a conductive grid that “short-circuits” the electrical component of the wave along the grooves, and allows the transverse components to pass through the polymeric bundle. The transmitted light is thus polarized in the transverse direction of the striatum.
By placing a second polarizing filter (called an analyzer) in the already polarized light, a shutter effect can be obtained.
When the orientation of the analyzer coincides with the plane of polarization of the incident light, all the light passes through, but for the orthogonal direction, the light is totally extinguished.
For intermediate positions there is partial passage of light, whose intensity varies according to Malus’s law :
I = Io Cos 2 (θ).
Light in a vacuum, like all electromagnetic waves, propagates with a speed c of approximately 300,000 km / s. But in a translucent medium its speed v is a little slower. The quotient between c and v is called the index of refraction of the translucent medium.
In some crystals, such as calcite, the refractive index is different for each polarization component. For this reason, when a light beam passes through a glass with birefringence, the beam is separated into two beams with linear polarization in orthogonal directions, as verified with a polarizer-analyzer filter.
Examples of light polarization
Light reflected from the surface of sea or lake water is partially polarized. The light from the blue sky, but not from the clouds, is partially polarized.
Some insects like the beetle C etonia aurata reflect light with circular polarization. The figure below shows this interesting phenomenon, in which the light reflected by the beetle can successively be observed without filters, with a right polarizing filter and then with a left polarizing filter.
In addition, a mirror has been placed that produces an image with an inverted state of polarization with respect to that of the light directly reflected by the beetle.
Applications of light polarization
Polarizing filters are used in photography to eliminate the flare produced by reflected light from reflective surfaces such as water.
They are also used to eliminate glare produced by partially polarized blue sky light, thus obtaining better contrast photographs.
In chemistry, as well as in the food industry, an instrument called a polarimeter is used , which allows to measure the concentration of certain substances that in solution produce a rotation of the polarization angle.
For example, through the passage of polarized light and with the help of a polarimeter, the sugar concentration in juices and beverages can be quickly determined to verify that it conforms to manufacturer’s standards and health controls.