The phenomena of interference and diffraction of light confirm its wave nature. At the beginning of the 19th century, T. Jung and O. Fresnel, having created the wave theory of light, considered light waves to be longitudinal, i.e. similar to sound waves. To do this, they had to introduce some kind of hypothetical environment called ether, in which the propagation of longitudinal light waves took place. At that time, it seemed incredible that light is transverse waves, since, by analogy with mechanical waves, one would have to assume that the ether is a solid body (transverse mechanical waves cannot propagate in a gaseous or liquid medium). However, already at that time there were facts contradicting the longitudinality of light waves.

Back in the Middle Ages, sailors brought unusual transparent stones from Iceland, which were later called Icelandic spar. Their unusualness lay in the fact that if a piece of Icelandic spar is put on any inscription, then through it the inscription will be seen bifurcated.

In 1669, the Danish scientist Bartholin reported interesting results from his experiments with Icelandic spar crystals. When passing through such a crystal, the beam splits into two (Fig. 2.6.1). These rays are named ordinary beam and extraordinary beam, and the phenomenon itself birefringence.

An ordinary ray obeys the ordinary law of refraction, and an extraordinary ray does not obey this law. The rays split in two even when they were normally incident on a crystal of Icelandic spar. If the crystal is rotated relative to the direction of the original beam, then both beams that have passed through the crystal are rotated. Bartholin also discovered that there is a certain direction in the crystal along which the incident beam does not split. However, he could not explain these phenomena.

A few years later, this Bartholin discovery attracted the attention of Huygens, who introduced the concept optical axis of the crystal(Bartolin actually discovered it).

The optical axis of the crystal called the selected direction in the crystal, along which the ordinary and extraordinary rays propagate without separating.

In 1809, the French engineer E. Malus conducted an experiment with tourmaline crystals (transparent greenish crystals). In this experiment, light was successively passed through two identical tourmaline plates. If the second plate is rotated relative to the first, then the intensity of the light passing through the second plate changes from the maximum value to zero (Fig. 2.6.2). Light intensity dependence I from the corner j between the optical axes of both plates has the form:

(Malus' law ), (2.6.1)

where I 0 is the intensity of the incident light.

Rice. 2.6.3 a. Rice. 2.6.3 b.

Neither double refraction nor Malus' law can be explained within the framework of the theory of longitudinal light waves. For longitudinal waves, the direction of propagation of the beam is the axis of symmetry. In a longitudinal wave, all directions in a plane perpendicular to the beam are equal.

To understand how a transverse wave behaves, consider a wave traveling along a cord in a vertical plane. If a box with a vertical slot is placed in the path of this wave (Fig. 2.6.3 a), then the wave passes freely through the slot. If the slot in the box is located horizontally, then the wave no longer passes through it (Fig. 2.6.3 b). This wave is also called plane polarized, because vibrations in it occur in one (vertical) plane.

Experiments with crystals of Icelandic spar and tourmaline made it possible to prove that the light wave is transverse. T. Jung (1816) was the first to suggest that light waves are transverse. Fresnel, independently of Jung, also put forward the concept of transverse light waves, substantiated it with numerous experiments and created the theory of birefringence of light in crystals.

In the mid-60s of the XIX century, Maxwell came to the conclusion that light is an electromagnetic wave. This conclusion was made on the basis of the coincidence of the speed of propagation of electromagnetic waves, which is obtained from Maxwell's theory, with the known value of the speed of light. By the time Maxwell concluded that electromagnetic waves existed, the transverse nature of light waves had already been proven experimentally. Therefore, Maxwell believed that the transverseness of electromagnetic waves is another important proof of the electromagnetic nature of light.

In the electromagnetic theory of light, the difficulties associated with the need to introduce a special medium for the propagation of waves - the ether, which had to be considered as a solid body, also disappeared.

In an electromagnetic wave, the vectors and are perpendicular to each other and lie in a plane perpendicular to the direction of wave propagation. It is accepted that the plane in which the vector oscillates is called vibration plane, and the plane in which the oscillations of the vector occur, plane of polarization. Since in all processes of interaction of light with matter the main role is played by the electric field strength vector, it is called light vector. If, during the propagation of an electromagnetic wave, the light vector retains its orientation, such a wave is called linearly polarized or plane polarized.

Linearly polarized light is emitted by lasers. However, light emitted from ordinary sources (such as sunlight, incandescent lamps, etc.) is not polarized. This is due to the fact that atoms emit light in separate trains independently of each other. As a result, the vector in the resulting light wave randomly changes its orientation in time, so that, on average, all directions of oscillations are equal.

A light wave in which the direction of oscillation of the light vector changes chaotically in time is called natural or unpolarized light.

Natural light, passing through a crystal of Icelandic spar or tourmaline, is polarized. The phenomenon of double refraction of light is explained by the fact that in many crystalline substances the refractive indices for two mutually perpendicularly polarized waves are different. Therefore, the crystal bifurcates the rays passing through it (Fig. 2.6.1). Two beams at the output of the crystal are linearly polarized in mutually perpendicular directions. Crystals in which birefringence occurs are called anisotropic.

Light can become polarized when reflected or scattered. In particular, the blue light of the sky is partially or completely polarized. The polarization of reflected light was first observed by Malus when he looked through a crystal of Icelandic spar at the reflection of the setting sun in the windows of the Luxembourg Palace in Paris. Malus found that the reflected light is polarized to some extent. The degree of polarization of the reflected beam depends on the angle of incidence: at normal incidence, the reflected light is not completely polarized, and when incident at an angle called the angle of full polarization or the Brewster angle, the reflected beam is 100% polarized. When reflected at the Brewster angle, the reflected and refracted rays are perpendicular to each other (Fig. 2.5.4). The reflected beam is plane-polarized parallel to the surface.

Because , and , then the Brewster angle is found by the formula .

Polarized light is widely used in many areas of technology (for example, for smooth control of light, in the study of elastic stresses, etc.). The human eye does not distinguish the polarization of light, but the eyes of some insects, such as bees, perceive it.

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Today in the lesson we will get acquainted with the phenomenon of polarization of light. Let us study the properties of polarized light. Let's get acquainted with the experimental proof of the transverseness of light waves.

The phenomena of interference and diffraction leave no doubt that propagating light has the properties of waves. But what kind of waves - longitudinal or transverse?

For a long time, the founders of wave optics, Jung and Fresnel, considered light waves to be longitudinal, i.e., similar to sound waves. At that time, light waves were considered as elastic waves in the ether that fills space and penetrates into all bodies. Such waves, it seemed, could not be transverse, since transverse waves can only exist in a solid body. But how can bodies move in solid ether without encountering resistance? After all, the ether should not impede the movement of bodies. Otherwise, the law of inertia would not hold.

However, gradually more and more experimental facts were accumulated, which could not be interpreted in any way, considering light waves to be longitudinal.

Experiments with tourmaline

And now, we will consider in detail only one of the experiments, very simple and extremely effective. This is an experiment with tourmaline crystals (transparent green crystals).

If a beam of light from an electric lamp or the sun is directed normally to such a plate, then the rotation of the plate around the beam will not cause any change in the intensity of the light that has passed through it (Fig. 1.). You might think that the light was only partially absorbed in the tourmaline and acquired a greenish color. Nothing else happened. But it's not. The light wave has acquired new properties.

These new properties are revealed if the beam is forced to pass through a second, exactly the same tourmaline crystal (Fig. 2(a)), parallel to the first. With identically directed axes of the crystals, again, nothing interesting happens: the light beam is simply further weakened due to absorption in the second crystal. But if the second crystal is rotated, leaving the first motionless, then an amazing phenomenon will be revealed - the extinguishing of light. As the angle between the axes increases, the light intensity decreases. And when the axes are perpendicular to each other, the light does not pass at all. It is completely absorbed by the second crystal.

A light wave that oscillates in all directions perpendicular to the direction of propagation is called natural.

Light in which the directions of oscillation of the light vector are somehow ordered is called polarized.

Light polarization- this is one of the fundamental properties of optical radiation (light), consisting in the inequality of different directions in a plane perpendicular to the light beam (the direction of propagation of the light wave).

Polarizers- devices that make it possible to obtain polarized light.

Analyzers- devices with which you can analyze whether the light is polarized or not.

Scheme of operation of the polarizer and analyzer

Transverse light waves

From the experiments described above, two facts follow:

firstly that the light wave coming from the light source is completely symmetrical with respect to the direction of propagation (during the rotation of the crystal around the beam in the first experiment, the intensity did not change).

Secondly that the wave emerging from the first crystal does not have axial symmetry (depending on the rotation of the second crystal relative to the beam, one or another intensity of the transmitted light is obtained).

Intensity of light coming out of the first polarizer:

Intensity of light passed through the second polarizer:

Intensity of light passing through two polarizers:

Let's conclude: 1. Light is a transverse wave. But in a beam of waves incident from a conventional source, there are oscillations of all possible directions, perpendicular to the direction of wave propagation.

2. Tourmaline crystal has the ability to transmit light waves with vibrations lying in one specific plane.

Model of linear polarization of a light wave

Polaroids

Not only tourmaline crystals are able to polarize light. The same property, for example, have the so-called polaroids. Polaroid is a thin (0.1 mm) film of herapatite crystals deposited on a celluloid or glass plate. With a polaroid, you can do the same experiments as with a tourmaline crystal. The advantage of polaroids is that you can create large surfaces that polarize light.

The downside of Polaroids is the purple tint they give to white light.

Diffraction and interference of light confirms the wave nature of light. But waves can be longitudinal and transverse. Consider the following experience.

Light polarization

Let us pass a beam of light through a rectangular tourmaline plate, one of the faces of which is parallel to the crystal axis. There were no visible changes. The light was only partially extinguished in the plate and acquired a greenish color.

picture

Now after we place another plate after the first one. If the axes of both plates are aligned, nothing will happen. But if the second crystal starts to rotate, then the light will be extinguished. When the axes are perpendicular, there will be no light at all. It will be completely absorbed by the second plate.

picture

Let's make two conclusions:

1. The wave of light is symmetrical with respect to the direction of propagation.

2. After passing through the first crystal, the wave ceases to have axial symmetry.

This cannot be explained from the point of view of longitudinal waves. Therefore, light is a transverse wave. The tourmaline crystal is a polaroid. It transmits light waves, the oscillations of which occur in one plane. This property is well illustrated in the following figure.

picture

Transverse light waves and electromagnetic theory of light

The light that is produced after passing through a polaroid is called plane polarized light. In polarized light, vibrations occur only in one direction - the transverse direction.

The electromagnetic theory of light originates in the work of Maxwell. In the second half of the 19th century, Maxwell theoretically proved the existence of electromagnetic waves that can propagate even in a vacuum.

And he suggested that light is also an electromagnetic wave. The electromagnetic theory of light is based on the fact that the speed of light and the speed of propagation of electromagnetic waves are the same.

By the end of the 19th century, it was finally established that light waves arise from the movement of charged particles in atoms. With the recognition of this theory, the need for a luminiferous ether, in which light waves propagate, has disappeared. light waves These are not mechanical, but electromagnetic waves.

Oscillations of a light wave consist of oscillations of two vectors: the intensity vector and the magnetic induction vector. The direction of oscillations of the electric field strength vector is considered to be the direction of oscillations in light waves.

transverse wave- a wave propagating in a direction perpendicular to the plane in which the particles of the medium oscillate (in the case of an elastic wave) or in which the vectors of the electric and magnetic fields lie (for an electromagnetic wave).

Transverse waves include, for example, waves in strings or elastic membranes, when the displacement of particles in them occurs strictly perpendicular to the direction of wave propagation, as well as plane homogeneous electromagnetic waves in an isotropic dielectric or magnet; in this case, transverse oscillations are performed by the vectors of electric and magnetic fields.

The transverse wave has polarization, i.e. its amplitude vector is oriented in a certain way in the transverse plane. In particular, linear, circular and elliptical polarizations are distinguished depending on the shape of the curve that the end of the amplitude vector describes. The concept of a transverse wave, as well as a longitudinal wave, is to some extent conditional and is associated with the way it is described. The "transversity" and "longitudinality" of the wave are determined by what quantities are actually observed. Thus, a plane electromagnetic wave can be described by a longitudinal Hertzian vector. In a number of cases, the division of waves into longitudinal and transverse ones generally loses its meaning. So, in a harmonic wave on the surface of deep water, the particles of the medium make circular motions in a vertical plane passing through the wave vector , i.e. particle oscillations have both longitudinal and transverse components.

In 1809, the French engineer E. Malus discovered a law named after him. In the experiments of Malus, light was sequentially passed through two identical plates of tourmaline (a transparent crystalline substance of a greenish color). The plates could rotate relative to each other through an angle φ

The transmitted light intensity turned out to be directly proportional to cos2 φ:

The Brewster phenomenon is used to create light polarizers, and the phenomenon of total internal reflection is used to spatially localize a light wave inside an optical fiber. The refractive index of the optical fiber material exceeds the refractive index of the environment (air), so the light beam inside the fiber experiences total internal reflection at the interface between the fiber and the medium and cannot go beyond the fiber. With the help of an optical fiber, it is possible to send a beam of light from one point in space to another along an arbitrary curvilinear trajectory.

At present, technologies have been created for the manufacture of quartz fibers with a diameter of , which practically do not have internal and external defects, and their strength is not less than that of steel. At the same time, it was possible to reduce the losses of electromagnetic radiation in the fiber to a value less than , and also significantly reduce the dispersion. This made it possible in 1988. put into operation a fiber-optic communication line that connected America with Europe along the bottom of the Atlantic Ocean. Modern FOCLs are capable of providing information transfer rates above .

At a high intensity of an electromagnetic wave, the optical characteristics of the medium, including the refractive index, cease to be constant and become functions of electromagnetic radiation. The principle of superposition for electromagnetic fields ceases to hold, and the medium is called non-linear. In classical physics, the model is used to describe nonlinear optical effects anharmonic oscillator. In this model, the potential energy of an atomic electron is written as a series in powers of displacement x of the electron relative to its equilibrium position