The degree of polarization of partially polarized light: definition, description and formula

Today we will reveal the essence of the wave nature of light and the phenomenon of "degree of polarization" associated with this fact.

Ability to see and light

The nature of light and its associated abilitysee worried human minds long ago. The ancient Greeks, trying to explain sight, assumed: either the eye emits certain "rays" that "feel" the surrounding objects and thereby inform the person of their appearance and form, or the things themselves radiate something that people catch and judge how everything is arranged . Theories were far from the truth: living beings see through reflected light. From the awareness of this fact to the ability to calculate what the degree of polarization is equal to, there was only one step left - to understand that light is a wave.

Light is a wave

A more detailed study of the light revealed:in the absence of interference, it spreads 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 are not interested. But as soon as the radiation collided with a transparent medium, amazing things were happening: the beam changed the direction of propagation and dimmed. In 1678, H. Huygens suggested that this can be explained by a single fact: light is a wave. The scientist formed the principle of Huygens, which was later supplemented by Fresnel. Due to this, today people know how to determine the degree of polarization.

The principle of Huygens-Fresnel

According to this principle, any point of the environment, up towhich reached the wave front, is a secondary source of coherent radiation, and the envelope of all the fronts of these points acts as a wave front at the next time instant. Thus, if the light propagates without interference, at each next moment the wave front will be the same as in the previous one. But it is worth the beam to meet an obstacle, as another factor comes into force: in dissimilar media, light travels at different speeds. Thus, the photon that managed to get to another medium first will spread in it faster than the last photon from the beam. Consequently, the wave front will tilt. The degree of polarization is not involved here at all, but it is simply necessary to fully understand this phenomenon.

Process time

It should be said separately that all these changeshappen incredibly fast. The speed of light in a vacuum is three hundred thousand kilometers per second. Any medium slows down the light, but not 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 even few devices can fix such short processes. So to understand the phenomenon is purely theoretical. Now, fully aware of what radiation is, the reader will want to understand how to find the degree of polarization of light? We will not deceive his expectations.

Polarization of light

We have already mentioned above that in different environments photonslights have different speeds. Since light is a transverse electromagnetic wave (not a condensation and rarefaction of the medium), it has two main characteristics:

• wave vector;
• amplitude (also a vector quantity).

The first characteristic indicates wherea beam of light is directed, and a polarization vector arises, that is, in which direction is the electric field intensity vector directed. This makes it possible to rotate around the wave vector. Natural light, for example, emitted from the sun, has no polarization. Oscillations are distributed in all directions with equal probability, there is no any chosen direction or figure along which the end of the wave vector oscillates.

Types of polarized light

Before learning to calculate the formula for the degree of polarization and to make calculations, it is worth understanding what kinds of polarized light are.

1. Elliptical polarization. The end of the wave vector of such light describes an ellipse.
2. Linear polarization. This is a special case of the first option. As the name implies, the picture is one direction.
3. Circular polarization. In another way, it is also called circular.

Any natural light can be represented asthe sum of two mutually perpendicularly polarized elements. It should be remembered that two perpendicularly polarized waves do not interact. Their interference is impossible, since from the point of view of the interaction of amplitudes, they seem not to exist for each other. When they meet, they simply pass on without changing.

Partially Polarized Light

The application of the polarization effect is immense.By directing natural light at the object, and having received a partially polarized, scientists can judge the properties of the surface. But how to determine the degree of polarization of partially polarized light?

There is a formula N.A. Umova:

P = (I_perch-AND_steam)/(AND_perch+ And_steam) where I_perch Is the intensity of the light in the direction perpendicular to the plane of the polarizer or the reflecting surface, and I_steam - parallel. The value of P can take values ​​from 0 (for natural light, devoid of any polarization) to 1 (for flat polarized radiation).

Can natural light be polarized?

The question at first glance is strange.After all, radiation, in which there are no specific directions, is called natural. However, for the inhabitants of the earth's surface, this is in a sense an approximation. The sun gives a stream of electromagnetic waves of various lengths. This radiation is not polarized. But passing through 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 magnitude is so small that it is often neglected. It is taken into account only in the case of accurate astronomical calculations, where the slightest error can add years to the star or the distance to our system.

Why is light polarized?

Above, we often said that in dissimilar environmentsphotons behave differently. But did not mention why. The answer depends on what kind of environment we are talking about, in other words, in what state of aggregation it is.

1. The medium is a crystalline body with strictlyperiodical structure. Usually the structure of such a substance is represented as a lattice with fixed balls - ions. But overall this is not entirely accurate. Such an approximation is often justified, but not in the case of interaction between the crystal and electromagnetic radiation. In fact, each ion oscillates around its equilibrium position, and not randomly, but in accordance with what its neighbors are, at what distances they are and how many are. Since all these vibrations are strictly programmed by a rigid medium, this ion can only emit an absorbed photon only in a strictly defined form. This fact gives rise to another: what the polarization of the outgoing photon will be depends on the direction in which it entered the crystal. This is called anisotropy of properties.
2. Среда – жидкость.Here the answer is more complicated, since there are two factors - the complexity of the molecules and fluctuations (thickening-rarefaction) 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 all of them are chaotic under normal conditions. However, the second factor (fluctuation) is capable of creating conditions under which a small number of molecules form in a small volume something like a temporary structure. At the same time, either all the molecules will be aligned, or they will be located relative to each other at some specific angles. If the light at this time passes through such a portion of the liquid, it will acquire partial polarization. From this it follows that temperature strongly influences the polarization of the liquid: the higher the temperature, the more serious the turbulence, and the more such areas will form. The latter conclusion is due to the theory of self-organization.
3. Среда – газ.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 slight polarization. And that is why the sky color is blue: the average size of compacted elements is such that the electromagnetic radiation of blue and violet colors is dissipated. 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 explore 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. But astronomers manage and give amazing photos of deep space to people.