Today we will reveal the essence of the wave nature of light and the phenomenon “degree of polarization” related to this fact.
The ability to see and light
The nature of light and the ability to see associated with it has worried human minds for a long time. The ancient Greeks, trying to explain vision, assumed: either the eye emits certain “rays” that “feel” the surrounding objects and thereby inform the person of their appearance and shape, or the things themselves emit something that people catch and judge how everything works. Theories turned out to be far from the truth: living beings see thanks to reflected light. From realizing this fact to being able to calculate what the degree of polarization is, there was one step left - to understand that light is a wave.
Light is a wave
With a more detailed study of the light, it turned out that in the absence of interference, it propagates in a straight line and does not turn anywhere. If an opaque obstacle gets in the way of the beam, then shadows are formed, and where the light itself goes, people were not interested. But as soon as the radiation collided with a transparent medium, amazing things happened: the beam changed directionspread and dimmed. In 1678, H. Huygens suggested that this can be explained by a single fact: light is a wave. The scientist formed the Huygens principle, which was later supplemented by Fresnel. Thanks to what people today know how to determine the degree of polarization.
Huygens-Fresnel principle
According to this principle, any point of the medium reached by the wave front is a secondary source of coherent radiation, and the envelope of all the fronts of these points acts as the wave front at the next moment of time. Thus, if light propagates without interference, at each next moment the wave front will be the same as at the previous one. But as soon as the beam meets an obstacle, another factor comes into play: in dissimilar media, light propagates at different speeds. Thus, the photon that managed to reach the other medium first will propagate in it faster than the last photon from the beam. Therefore, the wave front will tilt. The degree of polarization has nothing to do with it yet, but it is simply necessary to fully understand this phenomenon.
Process time
It should be said separately that all these changes are happening incredibly fast. The speed of light in a vacuum is three hundred thousand kilometers per second. Any medium slows down light, but not by much. The time during which the wave front is distorted when moving from one medium to another (for example, from air to water) is extremely short. The human eye cannot notice this, and few devices are capable of fixing such shortprocesses. So it is worth understanding the phenomenon purely theoretically. Now, fully aware of what radiation is, the reader will want to understand how to find the degree of polarization of light? Let's not deceive his expectations.
Polarization of light
We have already mentioned above that photons of light have different speeds in different media. Since light is a transverse electromagnetic wave (it is not a condensation and rarefaction of the medium), it has two main characteristics:
- wave vector;
- amplitude (also a vector quantity).
The first characteristic indicates where the light beam is directed, and the polarization vector arises, that is, in which direction the electric field strength vector is directed. This makes it possible to rotate around the wave vector. Natural light, such as that emitted by the sun, has no polarization. Oscillations are distributed in all directions with equal probability, there is no chosen direction or pattern along which the end of the wave vector oscillates.
Types of polarized light
Before you learn how to calculate the formula for the degree of polarization and make calculations, you should understand what types of polarized light are.
- Elliptical polarization. The end of the wave vector of such light describes an ellipse.
- Linear polarization. This is a special case of the first option. As the name implies, the picture is one direction.
- Circular polarization. In another way, it is also called circular.
Any natural light can be represented as the sum of two mutually perpendicular polarized elements. It is worth remembering that two perpendicularly polarized waves do not interact. Their interference is impossible, since from the point of view of the interaction of amplitudes, they do not seem to exist for each other. When they meet, they simply move on without changing.
Partly polarized light
The application of the polarization effect is huge. By directing natural light at an object, and receiving partially polarized light, scientists can judge the properties of the surface. But how do you determine the degree of polarization of partially polarized light?
There is a formula for N. A. Umov:
P=(Ilan-Ipar)/(Ilan+I par), where Itrans is the light intensity in the direction perpendicular to the plane of the polarizer or reflective surface, and Ipar- parallel. The P value can take values from 0 (for natural light devoid of any polarization) to 1 (for plane polarized radiation).
Can natural light be polarized?
The question is strange at first glance. After all, radiation in which there are no distinguished directions is usually called natural. However, for the inhabitants of the Earth's surface, this is in some sense an approximation. The sun gives a stream of electromagnetic waves of various lengths. This radiation is not polarized. But passingthrough a thick layer of the atmosphere, the radiation acquires a slight polarization. So the degree of polarization of natural light is generally not zero. But the value is so small that it is often neglected. It is taken into account only in the case of precise astronomical calculations, where the slightest error can add years to the star or distance to our system.
Why does light polarize?
We have often said above that photons behave differently in dissimilar media. But they didn't mention why. The answer depends on what kind of environment we are talking about, in other words, in what aggregate state it is.
- The medium is a crystalline body with a strictly periodic structure. Usually the structure of such a substance is represented as a lattice with fixed balls - ions. But in general, this is not entirely accurate. Such an approximation is often justified, but not in the case of the interaction of a crystal and electromagnetic radiation. In fact, each ion oscillates around its equilibrium position, and not randomly, but in accordance with what neighbors it has, at what distances and how many of them. Since all these vibrations are strictly programmed by a rigid medium, this ion is capable of emitting an absorbed photon only in a strictly defined form. This fact gives rise to another: what will be the polarization of the outgoing photon depends on the direction in which it entered the crystal. This is called property anisotropy.
- Wednesday - liquid. Here the answer is more complicated, since two factors are at work - the complexity of the molecules andfluctuations (condensation-rarefaction) of density. In itself, complex long organic molecules have a certain structure. Even the simplest molecules of sulfuric acid are not a chaotic spherical clot, but a very specific cruciform shape. Another thing is that under normal conditions they are all arranged randomly. However, the second factor (fluctuation) is able to create conditions under which a small number of molecules form in a small volume something like a temporary structure. In this case, either all molecules will be co-directed, or they will be located relative to each other at some specific angles. If light at this time passes through such a section of the liquid, it will acquire partial polarization. This leads to the conclusion that the temperature strongly affects the polarization of the liquid: the higher the temperature, the more serious the turbulence, and the more such areas will be formed. The last conclusion exists thanks to the theory of self-organization.
- Wednesday - gas. In the case of a homogeneous gas, polarization occurs due to fluctuations. That is why the natural light of the Sun, passing through the atmosphere, acquires a small polarization. And that is why the color of the sky is blue: the average size of the compacted elements is such that electromagnetic radiation of blue and violet colors is scattered. But if we are dealing with a mixture of gases, then it is much more difficult to calculate the degree of polarization. These problems are often solved by astronomers who study the light of a star that has passed through a dense molecular cloud of gas. Therefore, it is so difficult and interesting to study distant galaxies and clusters. Butastronomers are coping and giving amazing photos of deep space to people.
