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 met with two difficulties, which, as we have seen, Newton considered the decisive arguments against it: first, 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 "go 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 to straight lines. Only very accurate observations make it possible to notice the interference fringes of diffracting light parallel to the shadow boundaries.
The credit for creating the theory of diffraction belongs to Fresnel, later to Kirchhoff (1882), and later to Sommerfeld (1895). They mathematically analyzed these subtle phenomena and determined the limits in which the concept of a ray of light is applicable.
The second difficulty is related to the phenomena due to the polarization of light. Above, speaking of waves, we always had in mind longitudinal waves similar to known sound waves. Indeed, a sound wave consists of periodic densification and rarefaction, in which individual air particles move back and forth in the direction of wave propagation.
Transverse waves, of course, were also known: waves on the surface of water or vibrations of a stretched string, in which particles oscillate at right angles to the direction of wave propagation, can serve as an example. 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 movements of entire configurations (string oscillation). Neither observation nor the theory of wave propagation in elastic solids were not yet known. This explains the fact, which seems strange to us, that the recognition of optical waves as transverse vibrations required such a long time. Indeed, it is noteworthy that the impetus for the development of the mechanics of rigid elastic bodies was the experiments and concepts related to the dynamics of the imponderable and intangible ether.
Above (p. 91) we have explained the nature of polarization. Two rays emanating from a birefringent crystal of Icelandic spar do not behave, when passing through a second such crystal, like rays of ordinary light; namely, instead of a pair of equally intense beams, they give two beams of unequal intensity, one of which, under certain conditions, may even completely disappear.
In ordinary, "natural" light, different directions in the plane of the wave, i.e., in a plane perpendicular to the direction of the beam, are equal or equivalent (Fig. 62). In a beam of polarized light, for example, in one of the beams obtained by double refraction in a crystal of Icelandic spar, this is no longer the case. Malus discovered (1808) that polarization is not only a feature inherent in rays of light that have undergone double refraction in a crystal; this property can also be obtained by simple reflection. He was looking through a plate of Icelandic spar crystal at the setting sun reflected in the window. As he rotated his crystal, he noticed that the intensity of the two images of the sun was changing. This does not happen when looking 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 beam (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 preferable to another.
Saying that the reflected beam is polarized in the plane of incidence, they mean nothing more than the fact that such a beam behaves differently with respect to the second mirror, depending on the position relative to each other of the first plane of incidence and the second. The corpuscular theory cannot explain such properties, since light particles falling on a glass plate must either penetrate the plate or be reflected.
Two beams emanating from an Icelandic spar crystal are polarized in directions perpendicular to each other. If you direct them at the appropriate angle to the 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) in an attempt to obtain an interference pattern from two such beams polarized perpendicular to each other. Their attempt was unsuccessful. From this, Fresnel and Jung (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 oscillations of the ether particles are carried out 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 Ch. II, § 3), we have seen that its motion is determined in a unique way by specifying its rectangular coordinates, varying with time. Further, it is obvious that a birefringent crystal has the ability to transmit light vibrations at two different speeds in two mutually perpendicular directions. Hence, according to the Huygens principle, it follows that when such vibrations penetrate the crystal, they experience different deflections or are refracted in different ways, i.e., 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 planes
corresponding to each of the two outgoing beams are mutually perpendicular (Fig. 64). Two such oscillations, obviously, cannot influence each other - they cannot interfere. Now, if the polarized beam re-enters the second crystal, it is transmitted without attenuation only if the direction of its oscillations has the correct orientation relative to the crystal - such that this oscillation can propagate without interference.
Fig. 64. Two beams 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 is polarized. It carries vibrations that occur in only one direction.
In all other positions, the beam splits into two, and the intensity of the two resulting beams varies depending on the orientation of the second crystal.
Similar conditions hold for reflection. If the reflection occurs at an appropriate angle, then of the two vibrations, one of which is parallel and the other is 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 - 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 see now, gave rise to a number of deep studies, theories and discussions.
And O. Fresnel knew that light waves are longitudinal, that is, they are similar to sound waves. At that time, light waves were perceived as elastic waves in the ether, which fill all space and penetrate into every body. It seemed that the waves could not be called transverse.
But still, little by little, more and more experimental evidence and facts were accumulated that could not be explained by assuming that light waves are longitudinal. After all transverse waves could only exist in solids. But how can a body move in solid ether without resistance? The ether should not slow down the movement of bodies in any way. After all, otherwise it would not be fulfilled.
One simple and useful experiment with a tourmaline crystal may be considered. It is transparent and has a green color.
The tourmaline crystal has this crystal. This crystal is classified as a uniaxial crystal. A rectangular plate of tourmaline is taken, cut out so that one of its faces is parallel to the axis of the crystal itself. If a beam of electric or sunlight is directed normally at this plate, then the rotation of the plate around it will not cause a change in the intensity of the light that passes through it. There is a feeling that the transmitted light in the tourmaline was partially absorbed and acquired a light green color. Nothing else happens. But this is wrong. The wave of light acquires new properties.
They can be detected if a beam of light passes through the same second tourmaline crystal, which is parallel to the first. With the same direction of the axes of the two crystals, nothing interesting happens either, only the light beam is more and more weakened due to absorption, passing through the second crystal. But when the second crystal rotates, if the first one is left motionless, an interesting phenomenon called "extinguishing of light" will be revealed. As the angle between these two axes increases, the saturation of the transmitted light beam decreases. When two axes are perpendicular to one another, light cannot pass at all. It will be completely absorbed by the second crystal. How is this explained?
Transverse light waves
From the description of the facts shown earlier, it follows:
1. Firstly, the light wave that comes from the light source is absolutely symmetrical with respect to the direction in which it propagates. During the revolution of this crystal around the passing beam of light during the first experiment, its intensity did not change.
2. Secondly, the wave coming out of the first crystal will not have axial symmetry. The intensity of light passing through another crystal depends on its rotation.
Longitudinal waves are completely symmetrical with respect to the direction of propagation. Oscillations of longitudinal waves occur along such a direction, this oscillation is a wave. That is why it is not possible to explain the experiment with the rotation of the second crystal, assuming that the light wave is longitudinal, because these are transverse waves.
One can fully explain the experience by making two assumptions:
Assumption number one applies directly to light: light waves are transverse waves. But in a beam of light waves incident from a light source, there are oscillations of various directions, which are perpendicular to the direction along which such a wave propagates. In this case, considering such an assumption, we can conclude that the light wave has at the same time being transverse. For example, the waves water surface they do not have such symmetry, because the vibrations of water particles occur exclusively in the vertical plane.
Waves of light with vibrations in different directions, which are perpendicular to the directions of propagation, are called natural. This name is justified because standard conditions different light sources create just such waves. This assumption is explained by the results of the first experiment. The rotation of the tourmaline crystal does not change the saturation of the transmitted beam of light, because this incident wave has axial symmetry, even though it is a transverse wave.
The second assumption concerns the crystal itself. Tourmaline has the ability to transmit light waves with vibrations that occur in a certain plane. This light is called polarized (or plane polarized). It differs from natural, unpolarized.
This assumption is explained by the second experiment. Plane polarized light (wave) emerges from the first tourmaline crystal. When the crystals cross at an angle of ninety degrees, the wave cannot pass through the second of them. If the crossing angle is different, then they will pass through which will be equal to the projection of the amplitude of the wave that passed through the first plate in the direction of the axis of the second. This is precisely the proof of the theory that light waves are transverse 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 field(for an electromagnetic wave).
Transverse waves include, for example, waves in strings or elastic membranes, when particle displacements in them occur 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 successively passed through two identical tourmaline plates (transparent crystalline substance 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 is greater than the refractive index environment(air), therefore, 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 losses electromagnetic radiation in the fiber to a value less than , and significantly reduce the dispersion. This made it possible in 1988. put into operation a fiber-optic communication line connecting along the seabed Atlantic Ocean America with Europe. 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
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 fluctuates in the transverse mechanical wave? 5. What type of waves is a sound wave? 6. What type of waves is an electromagnetic wave? Why?
In 1865, Maxwell concluded that light is an electromagnetic wave. One of the arguments in favor of this statement is the coincidence of the speed 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 of various directions perpendicular to the direction of wave propagation. A light wave that oscillates in all directions perpendicular to the direction of propagation is called a natural wave.
Polarized light A 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 It is a thin (0.1 mm) film of herapatite crystals deposited on a celluloid or glass plate. Transparent films (polymer, single-crystal, etc.) that convert unpolarized light into linearly polarized, because transmit light in 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 part of it is reflected, and part is refracted and propagated in the second medium. By placing an analyzer (for example, tourmaline) in the path of the reflected and refracted beams, one can make sure that the reflected and refracted beams are partially polarized: when the analyzer is rotated around the beams, the light intensity periodically increases and decreases (complete extinction is not observed!). Further studies have shown that in the reflected beam, oscillations perpendicular to the plane of incidence prevail (in the figure they are indicated by dots), in the refracted beam - oscillations parallel to the plane of incidence (shown by arrows).
Experimental verification of the polarization of light emitted by various sources A liquid crystal monitor produces polarized light. When you turn the polarizer, it is weakened, when you turn it by 90, it is completely extinguished. The radiation of the calculator display is also polarized. Polarized mobile phone display light. Light reflected from glass is polarized. Look at the glass through a polaroid. By rotating the polaroid, we achieve the disappearance of glare.
Polarized light in nature Polarized reflected light, glare, for example, lying on the surface of water, Scattered light from the sky is nothing but sunlight, which 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, no worse than the Vikings, use this ability to orient themselves in cases where the Sun is covered by clouds. The light of some astronomical objects is polarized. Most famous example The Crab Nebula in the constellation Taurus. Some species of beetles that have metallic sheen, turn the light reflected from their back into a circularly polarized one. This is the name of polarized light, the plane of polarization of which is twisted in space in a helical direction, to the left or to the right.
Polarized and Anti-Glare Sunglasses Driving safely at night, day, dusk, fog and winter. Polarized lenses remove glare from the windshield, from wet roads, from snow, protect from the headlights of oncoming cars, relieve fatigue, and improve visibility in any weather. They are indispensable for polar explorers, who constantly have to look at the dazzling reflection of the sun's rays from the icy snow field.
Detection of stresses in transparent bodies (defectoscopy): If stresses appear in a transparent material (caused by internal stresses or an external load), then the material begins to rotate the angle of polarization inhomogeneously. This effect is stronger in polymers than in glass. EXPERIENCE: Clamp the transparent plastic box from a CD between two polaroids. Light experiences non-uniform polarization, which manifests itself in different intensities of light passing through polarizers, coloring the field of view in different colours in transmitted light. When the box is bent or compressed, the intensity of the transmitted light changes, and the color of the light transmitted through the polaroids also changes. This is how stresses are detected in transparent samples.
Obtaining a stereo image, stereo monitor To obtain the effect of volume (stereo effect), it is necessary to show each eye its own picture, as if different eyes look 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 a different direction 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 substances that, under the influence of voltage, turn into electric field. Liquid crystals have anisotropic properties. In particular, depending on the orientation, they reflect and transmit light in different ways, and rotate its plane of polarization. The TFT panel is like a sandwich sandwich. The liquid crystal layer is located between two polarizing panels. The voltage causes the crystals to act like a shutter, blocking or letting light through. The intensity of light passing through a polarizer depends on the voltage.
Conclusions: Tourmaline crystal (polaroid) converts natural light into plane polarized. Polarization is one of the wave properties of light. Various light sources can emit both polarized and unpolarized light. With the help of polaroids, you can control the intensity of light; The phenomenon of light polarization occurs in nature and is widely used in modern technology. Light is a transverse wave.
