What phenomenon proves the transverse nature of light waves. Transverse light waves

Although the phenomenon of interference hardly admits of any other interpretation than on the basis of the wave theory, the general acceptance of this theory was met with two difficulties which, as we have seen, Newton considered decisive arguments against it: firstly, the rectilinear propagation of light in general case and, secondly, the nature of the polarization phenomenon. The first difficulty was overcome within the framework of the wave theory itself, when it reached sufficient level development: has been established; that waves “bend around corners,” but only in regions of the order of the wavelength. Since the latter are extremely small in the case of light, it appears to the naked eye that the shadows have sharp boundaries, and the rays are limited by straight lines. Only very precise observations allow one to notice interference fringes of diffracting light parallel to the boundaries of the shadow.

The honor of creating the theory of diffraction belongs to Fresnel, later to Kirchhoff (1882), and later to Sommerfeld (1895). They analyzed these subtle phenomena mathematically and determined the limits within which the concept of a ray of light was applicable.

The second difficulty is associated with phenomena caused by the polarization of light. Above, when speaking about waves, we always meant longitudinal waves, similar to the well-known sound waves. Really, sound wave consists of periodic compactions and rarefactions in which individual air particles move back and forth in the direction of wave propagation.

Transverse waves, of course, were also known: an example would be waves on the surface of water or oscillations of a stretched string, in which the particles vibrate at right angles to the direction of propagation of the wave. But in these cases we are not dealing with waves inside the substance, but either with phenomena on the surface (waves on water), or with the movements of entire configurations (vibration of a string). Neither observations nor the theory of wave propagation in elastic solids were known at that time. This explains the strange fact that it seems to us that the recognition of optical waves as transverse oscillations took so long. Indeed, it is noteworthy that the impetus for the development of the mechanics of solid elastic bodies came from experiments and concepts related to the dynamics of the weightless and intangible ether.

Above (p. 91) we explained the nature of polarization. Two rays emanating from a birefringent crystal of Iceland spar do not behave like rays of ordinary light when passing through a second such crystal; namely, instead of a pair of equally intense rays, they produce two rays of unequal intensity, one of which, under certain conditions, can even disappear completely.

In ordinary, “natural” light, different directions in the plane of the wave, that is, in the plane perpendicular to the direction of the beam, are equal, or equivalent (Fig. 62). In a ray of polarized light, for example in one of the rays resulting from double refraction in an Iceland spar crystal, this is no longer the case. Malus discovered (1808) that polarization is a feature inherent not only in rays of light that have undergone double refraction in a crystal; this property can also be obtained by simple reflection. He looked through the plate of Iceland spar crystal at the setting sun reflected in the window. As he turned his crystal, he noticed that the intensity of the two images of the sun was changing. This does not happen if you look through such a crystal directly at the sun. Brewster (1815) showed that light reflected from a glass plate at a certain angle is reflected from a second such plate to a different extent if the latter is rotated around the incident ray (Fig. 63). The plane perpendicular to the surface of the mirror in which the incident and reflected rays lie is called the plane of incidence.

Fig. 62. In a beam of natural light, no direction perpendicular to the plane of propagation is preferred over another.

When we say that the reflected beam is polarized in the plane of incidence, we mean nothing more than the fact that such a beam behaves differently in relation to the second mirror depending on the position of the first plane of incidence and the second relative to each other. The corpuscular theory cannot explain such properties, since particles of light falling on a glass plate must either penetrate into the plate or be reflected.

Two beams emanating from an Iceland spar crystal are polarized in directions perpendicular to each other. If you point them at the appropriate angle at a mirror, then one of them will not be reflected at all, while the other will be completely reflected.

Fresnel and Arago performed a decisive experiment (1816), attempting to obtain an interference pattern from two such rays polarized perpendicular to each other. Their attempt was unsuccessful. From here Fresnel and Young (1817) made the final conclusion that light vibrations must be transverse.

Fig. 63. To the experiment on polarization. If you rotate the first or second plate around the incident beam as an axis, the intensity of the reflected beam changes.

In fact, this conclusion immediately makes clear the unusual behavior of polarized light. The ether particles vibrate not in the direction of wave propagation, but in a plane perpendicular to this direction - in the plane of the wave (Fig. 62). But any movement of a point in a plane can be considered as consisting of two movements in two mutually perpendicular directions. Considering the kinematics of a point (see Chapter II, § 3), we saw that its movement is determined uniquely by specifying its rectangular coordinates, which vary depending on time. It is further evident that a birefringent crystal has the ability to transmit light vibrations at two different speeds in two mutually perpendicular directions. From here, according to Huygens' principle, it follows that when such vibrations penetrate a crystal, they experience different deviations or are refracted in different ways, that is, they are separated in space. Each ray emerging from the crystal thus consists only of oscillations in a certain plane passing through the direction of the ray, and the plane

corresponding to each of the two outgoing rays, mutually perpendicular (Fig. 64). Two such oscillations obviously cannot affect each other - they cannot interfere. Now, if the polarized beam again hits the second crystal, it is transmitted without attenuation only if the direction of its vibration is in the correct orientation relative to the crystal - one in which this vibration can propagate without interference.

Fig. 64. Two rays resulting from double refraction are polarized perpendicular to each other.

Fig. 65. Reflection of a ray incident on a surface at the Brewster angle. At a certain angle of incidence a, the reflected beam turns out to be polarized. It carries vibrations that occur in only one direction.

In all other positions, the beam is split into two, and the intensity of the two resulting beams varies depending on the orientation of the second crystal.

Similar conditions apply to reflection. If reflection occurs at the appropriate angle, then of two vibrations, one of which is parallel and the other perpendicular to the plane of incidence, only one is reflected; the other penetrates the mirror, being absorbed in the case of a metal mirror or passing through in the case of a glass plate (Fig. 65). Which of the two vibrations is perpendicular?

or parallel to the plane of incidence - it turns out to be reflected, of course, it is impossible to establish. (In Fig. 65 it is assumed that the second option is being implemented.) However, this question of the orientation of the oscillations relative to the plane of incidence or the direction of polarization, as we will now see, has given rise to a number of in-depth studies, theories and discussions.

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 electric and electric vectors lie magnetic field(for electromagnetic wave).

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

The transverse wave is polarized, 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, like a longitudinal wave, is to some extent arbitrary and is associated with the method of its description. The “transverse” and “longitudinal” of the wave are determined by what quantities are actually observed. Thus, a plane electromagnetic wave can be described by a longitudinal Hertz vector. In some cases, dividing waves into longitudinal and transverse ones loses its meaning altogether. Thus, in a harmonic wave on the surface of deep water, particles of the medium perform circular motions in a vertical plane passing through the wave vector, i.e. particle vibrations have both longitudinal and transverse components.

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

The intensity of the transmitted light 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 environment(air), therefore the light beam inside the fiber experiences total internal reflection at the fiber-medium interface and cannot go beyond the fiber. Using an optical fiber, you can send a beam of light from one point in space to another along an arbitrary curved path.

Currently, technologies have been created for the production of quartz fibers with a diameter of , which have virtually no internal or external defects, and their strength is no less than the strength of steel. At the same time, it was possible to reduce losses electromagnetic radiation in the fiber to a value less than , and also significantly reduce dispersion. This allowed in 1988. put into operation a fiber-optic communication line connecting along the bottom Atlantic Ocean America and Europe. Modern fiber-optic lines are capable of providing information transmission speeds exceeding .

At high intensity of the 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 true, and the medium is called nonlinear. 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

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

Polarization of light

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

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Now let's 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, the light will go out. When the axes are perpendicular, there will be no light at all. It will be completely absorbed by the second plate.

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Let's draw two conclusions:

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

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

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

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Transverseness of light waves and electromagnetic theory of light

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

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 disappeared. Light waves- these are not mechanical, but electromagnetic waves.

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

The phenomena of interference and diffraction of light confirm its wave nature. IN early XIX 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 a certain hypothetical environment called ether, in which the propagation of longitudinal light waves occurred. At that time it seemed incredible that light is transverse waves, since by analogy with mechanical waves one would have to assume that ether is solid(transverse mechanical waves cannot propagate in a gaseous or liquid medium). However, already at that time there were facts that contradicted the longitudinality of light waves.

Back in the Middle Ages, sailors brought unusual transparent stones from Iceland, which were later called Iceland spar. Their unusual feature was that if a piece of Iceland spar was placed on any inscription, then through it the inscription would be seen as forked.

In 1669, the Danish scientist Bartholin reported interesting results from his experiments with Iceland 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 ray, and the phenomenon itself - birefringence.

An ordinary ray obeys the ordinary law of refraction, but an extraordinary ray does not obey this law. The rays bifurcated even when falling normally onto an Iceland spar crystal. If the crystal is rotated relative to the direction of the original ray, then both rays that passed through the crystal are rotated. Bartolin also discovered that there is a certain direction in the crystal along which the incident beam does not bifurcate. However, he could not give an explanation for these phenomena.

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

Optical axis of the crystal is the selected direction in the crystal along which ordinary and extraordinary rays propagate without being separated.

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 was rotated relative to the first, then the intensity of the light passing through the second plate changed from the maximum value to zero (Fig. 2.6.2). Dependence of light intensity I from the corner j between the optical axes of both plates has the form:

(Malus's law ), (2.6.1)

Where I 0 – intensity of incident light.

Rice. 2.6.3 A. Rice. 2.6.3 b.

Neither birefringence nor Malus's law can be explained within the framework of the theory of longitudinal light waves. For longitudinal waves, the direction of beam propagation 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 you place a box with a vertical slit in the path of this wave (Fig. 2.6.3 A), then the wave passes freely through the slit. If the gap 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 iceland spar and tourmaline crystals made it possible to prove that the light wave is transverse. The first guess about the transverseness of light waves was made by T. Jung (1816). Fresnel, independently of Young, also put forward the concept of transverse light waves, substantiated it with numerous experiments and created the theory of double refraction of light in crystals.

In the mid-60s of the 19th century, Maxwell came to the conclusion that light is an electromagnetic wave. This conclusion was made based on the coincidence of the speed of propagation of electromagnetic waves, which is obtained from Maxwell’s theory, with known value speed of light. By the time Maxwell concluded about the existence of electromagnetic waves, the transverse nature of light waves had already been proven experimentally. Therefore, Maxwell believed that the transversality 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 wave propagation, 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 propagation of the wave. It is customary to call the plane in which the vector oscillates plane of oscillation, and the plane in which the vector oscillates, 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 by conventional sources (e.g. sunlight, radiation from incandescent lamps, etc.), 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 directions of oscillations of the light vector change chaotically in time is called natural or unpolarized light.

Natural light passing through a crystal of Iceland spar or tourmaline becomes polarized. The phenomenon of birefringence 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). The 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, blue light from the sky is partially or fully polarized. Polarization of reflected light was first observed by Malus when he looked through an Iceland spar crystal at the reflection of the setting sun in the windows of the Luxembourg Palace in Paris. Malus established that the reflected light is polarized to one degree or another. The degree of polarization of the reflected beam depends on the angle of incidence: at normal incidence the reflected light is completely unpolarized, but when incident at an angle called the angle of complete polarization or Brewster's 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 fields of technology (for example, for smooth adjustment 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.

Let's answer the questions: 1. What two types are all waves divided into? 2. What waves are called longitudinal? 3. What waves are called transverse? 4. What oscillates transversely mechanical wave? 5. What type of waves is a sound wave? 6. What type of wave is an electromagnetic wave? Why?

In 1865, Maxwell came to the conclusion that light is an electromagnetic wave. One of the arguments in favor of this statement is the coincidence of the speed of electromagnetic waves, theoretically calculated by Maxwell, with the speed of light determined experimentally (in the experiments of Roemer and Foucault).

Natural light Light is a transverse wave. In a beam of waves incident from a conventional source, there are oscillations in all possible directions, perpendicular to the direction of propagation of the waves. A light wave that oscillates in all directions perpendicular to the direction of propagation is called natural.

Polarized light Tourmaline crystal has the ability to transmit light waves with vibrations lying in one specific plane. Such light is called polarized or, more precisely, plane-polarized, in contrast to natural light, which can also be called unpolarized.

Polaroid is a thin (0.1 mm) film of herapatite crystals applied to celluloid or glass plate. Transparencies(polymer, monocrystalline, etc.), converting unpolarized light into linearly polarized light, because They transmit light from only one direction of polarization. Polaroids were invented by the American scientist E. Land in 1932.

If natural light falls on the interface between two dielectrics (for example, air and glass), then some of it is reflected, and some is refracted and propagated in the second medium. By installing an analyzer (for example, tourmaline) in the path of the reflected and refracted rays, you can make sure that the reflected and refracted rays are partially polarized: when the analyzer is rotated around the rays, the light intensity periodically intensifies and weakens (complete quenching is not observed!). Further studies showed that in the reflected beam, vibrations perpendicular to the plane of incidence predominate (they are indicated by dots in the figure), while in the refracted beam, vibrations parallel to the plane of incidence (depicted by arrows) predominate.

Experimentally testing the polarization of light emitted by various sources. A liquid crystal monitor produces polarized light. When the polarizer is turned, it weakens; when turned by 90, it is completely extinguished. The emission from the calculator display is also polarized. Display light polarized mobile phone. Light reflected from glass is polarized. Look at the glass through a Polaroid. By rotating the polaroid we make the glare disappear.

Polarized light in nature Reflected light, glare, for example, lying on the surface of water is polarized. Scattered light from the sky is nothing more than sunlight that has undergone multiple reflections from air molecules, refracted in water droplets or ice crystals. Therefore, in a certain direction from the Sun it is polarized. Many insects, unlike humans, see the polarization of light. Bees and ants use this ability to navigate when the Sun is obscured by clouds. The light of some astronomical objects is polarized. Most famous example– The Crab Nebula in the constellation Taurus. Some species of beetles, which have a metallic sheen, convert light reflected from their backs into circularly polarized light. This is the name for polarized light, the plane of polarization of which is twisted in space in a helical manner, to the left or to the right.

Polarized and anti-glare sunglasses Safe driving at night, day, twilight, fog and winter. Polarized lenses reduce glare from windshield, from wet roads, from snow, protect from the headlights of oncoming cars, relieve fatigue, improve visibility in any weather. They are indispensable for polar explorers who constantly have to look at a dazzling reflection sun rays from an icy snow field.

Obtaining a stereo image, stereo monitor To obtain a volume effect (stereo effect), it is necessary to show each eye its own picture, as if different eyes were looking at the object from different angles; everything else our brain will complete and calculate on its own. In a stereo monitor, even and odd rows of pixels on the screen must have different directions of light polarization. The lenses of the glasses are polarizers, rotated 90 degrees relative to each other - only even lines are visible through one lens of the glasses, and odd lines through the other. Each eye will see only the picture that is intended for it, so the image becomes three-dimensional.

The principle of operation of LCD displays The operation of LCD displays is based on the phenomenon of polarization of the light flux. Liquid crystals are organic matter, capable of rotating under tension electric field. Liquid crystals have anisotropy properties. In particular, depending on the orientation, they reflect and transmit light differently and rotate its plane of polarization. A thin-film transistor panel looks like a multilayer sandwich. A layer of liquid crystals is located between two polarizing panels. The voltage causes the crystals to act like a shutter, blocking or allowing light through. The intensity of light passing through the polarizer depends on the voltage.

Conclusions: Tourmaline crystal (Polaroid) converts natural light into plane-polarized light. Polarization is one of the wave properties of light. Various sources Lights can emit both polarized and unpolarized light. Polaroids can be used to control the light intensity; The phenomenon of polarization of light occurs in nature and is widely used in modern technology. Light is a transverse wave. cbbb15dd9463b3/gDM

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