Today we will reveal the essence of the wave nature of light and the phenomenon of the degree of polarization associated with this fact.
Ability to see and light
The nature of light and its ability to see has been exciting human minds for a long time. The ancient Greeks, trying to explain their vision, suggested: either the eye emits some “rays” that “feel” the surrounding objects and thereby tell the person their appearance and shape, or things themselves emit something that people catch and judge how everything works . Theories were far from the truth: living things see through reflected light. From 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 propagates in a straight line and does not turn anywhere. If an opaque obstacle gets in the way of the beam, then shadows form, 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 the direction of propagation and faded. 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 which people today know how to determine the degree of polarization.
Huygens-Fresnel principle
According to this principle, any point of the medium to which the wave front has reached 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 instant in time. Thus, if the light propagates without interference, at each next moment the wave front will be the same as in the previous one. But once the beam encounters an obstacle, another factor takes effect: in dissimilar environments, light travels at different speeds. Thus, the photon that managed to get to another medium first will propagate in it faster than the last photon from the beam. Consequently, the wave front will tilt. The degree of polarization has nothing to do with it, but to fully understand this phenomenon is absolutely necessary.
Process time
It is worth mentioning separately that all these changes occur incredibly quickly. The speed of light in a vacuum is three hundred thousand kilometers per second. Any medium slows down the light, but not by much. The time during which the wave front is distorted during the transition from one medium to another (for example, from air to water) is extremely short. The human eye cannot notice this, and few devices can record such short processes. So 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? Let's not deceive his expectations.
Light polarization
We have already mentioned above that in different environments photons of light have different speeds. Since light is a transverse electromagnetic wave (it is not a condensation or rarefaction of a medium), it has two main characteristics:
- wave vector;
- amplitude (also a vector quantity).
The first characteristic indicates where the light beam is directed, and a polarization vector occurs, that is, in which direction the electric field vector is directed. This makes it possible to rotate around the wave vector. Natural light, for example, emitted by the Sun, has no polarization. Oscillations are distributed in all directions with equal probability; there is no chosen direction or figure 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, it is worthwhile to 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 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 do not seem to exist for each other. When they meet, they simply pass on without changing.
Partially Polarized Light
The application of the polarization effect is huge. By sending natural light to the object, and having received 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 of N.A. Umova:
P = (I per -I pair ) / (I per + I pair ), where I per is the light intensity in the direction perpendicular to the plane of the polarizer or reflective surface, and I pair is parallel. The value of P 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 any distinguished directions is 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 passing through a thick layer of the atmosphere, the radiation acquires a slight polarization. So the degree of polarization of natural light as a whole is not equal to zero. But the value 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 a star to years or the distance to our system.
Why is light polarized?
We often said above that in dissimilar environments, photons behave differently. But they 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.
- A 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 overall, this is not entirely accurate. Such an approximation is often justified, but not in the case of the interaction of the crystal and electromagnetic radiation. In fact, each ion oscillates around its equilibrium position, not chaotically, but in accordance with what neighbors it has, at what distances and how many there are. 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 emerging photon depends on the direction in which it entered the crystal. This is called anisotropy of properties.
- The medium is liquid. Here the answer is more complicated, since two factors act - the complexity of the molecules and fluctuations (thickening-rarefaction) of the density. Complex long organic molecules in themselves have a definite structure. Even the simplest molecules of sulfuric acid are not a chaotic spherical clot, but a very specific cross-shaped shape. Another thing is that they are all under normal conditions randomly located. However, the second factor (fluctuation) is able to create conditions under which a small number of molecules in a small volume form something like a temporary structure. In this case, either all molecules will be codirectional, or they will be located relative to each other at certain specific angles. If light at this time passes through such a section of liquid, it will acquire partial polarization. It follows from this that the temperature strongly affects the polarization of the liquid: the higher the temperature, the more severe the turbulence, and the more such sections will form. The last conclusion is due to the theory of self-organization.
- Wednesday is 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 the 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 passing 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.