Electromagnetic waves. The concept of an electromagnetic wave

These phenomena are inherent in waves of any nature. Moreover, the phenomena of interference, diffraction, and polarization are characteristic only of wave processes and can only be explained on the basis of wave theory.

Reflection and refraction. The propagation of waves is geometrically described using rays. In a homogeneous environment ( n= const) the rays are rectilinear. However, at the interface between the media, their directions change. In this case, two waves are formed: reflected, propagating in the first medium with the same speed, and refracted, propagating in the second medium with a different speed, depending on the properties of this medium. The phenomenon of reflection is known for both sound (echo) and light waves. Due to the reflection of light, an imaginary image is formed in the mirror. The refraction of light underlies many interesting atmospheric phenomena. It is widely used in various optical devices: lenses, prisms, optical fibers. These devices are elements of devices for various purposes: cameras, microscopes and telescopes, periscopes, projectors, optical systems communications, etc.

Interference waves - the phenomenon of energy redistribution when two (or several) coherent (matched) waves are superimposed, accompanied by the appearance of an interference pattern of alternating maxima and minima of the intensity (amplitude) of the resulting wave. Waves are called coherent, for which the phase difference at the point of addition remains unchanged in time, but can change from point to point and in space. If the waves meet "in phase", i.e. simultaneously reach the maximum deviation in one direction, then they reinforce each other, and if they meet "in antiphase", i.e. simultaneously achieve opposite deviations, then weaken each other. Coordination of the oscillations of two waves (coherence) of two waves in the case of light is possible only if they have a common origin, which is due to the peculiarities of the radiation processes. The exception is lasers, whose radiation is characterized by high coherence. Therefore, to observe interference, light coming from one source is divided into two groups of waves, either passing through two holes (slits) in an opaque screen, or due to reflection and refraction at the interface of media in thin films. Interference pattern from a monochromatic source ( λ=const) on the screen for rays passing through two narrow closely spaced slits, has the form of alternating bright and dark stripes (Jung's experiment, 1801). Bright stripes - intensity maxima are observed at those points of the screen where the waves from two slots meet "in phase", i.e. their phase difference

, m =0,1,2,…,(3.10)

This corresponds to the difference in the path of the rays, a multiple of an integer number of wavelengths λ

, m =0,1,2,…,(3.11)

Dark bars (mutual repayments), i.e. intensity minima occur at those points of the screen at which the waves meet "out of phase", i.e., their phase difference is

, m =0,1,2,…,(3.12)

This corresponds to the difference in the path of the rays, a multiple of an odd number of half-waves

, m =0,1,2,….(3.13)

Interference is observed for different waves. Interference of white light including all waves visible light in the wavelength range micron can appear as iridescent coloration of thin films of gasoline on the surface of the water, soap bubbles, oxide films on the surface of metals. Interference maximum conditions in different points films are made for different wavelengths with different wavelengths, which leads to the amplification of waves of different colors. The interference conditions are determined by the wavelength, which for visible light is a fraction of a micron (1 μm = 10 -6 m), so this phenomenon underlies various precision (“high-precision”) methods of research, control and measurement. The use of interference is based on the use of interferometers, interference spectroscopes, as well as the holography method. Light interference is used to measure the wavelength of radiation, study the fine structure of spectral lines, determine densities, refractive indices of substances, and the thickness of thin coatings.

Diffraction- a set of phenomena that occur during the propagation of a wave in a medium with a pronounced inhomogeneity of properties. This is observed when waves pass through a hole in the screen, near the border of opaque objects, etc. Diffraction causes the wave to wrap around an obstacle whose dimensions are commensurate with the wavelength. If the size of the obstacle is much larger than the wavelength, then the diffraction is weak. On macroscopic obstacles, diffraction of sound, seismic waves, radio waves is observed, for which 1 cm km. To observe the diffraction of light, the obstacles must be substantially smaller. Diffraction sound waves explains the ability to hear the voice of a person around the corner of the house. The diffraction of radio waves around the Earth's surface explains the reception of radio signals in the range of long and medium radio waves far beyond the line of sight of the emitting antenna.

The diffraction of waves is accompanied by their interference, which leads to the formation of a diffraction pattern, alternating intensity maxima and minima. When light passes through a diffraction grating, which is a set of alternating parallel transparent and opaque bands (up to 1000 per 1 mm), a diffraction pattern appears on the screen, the position of the maxima of which depends on the radiation wavelength. This makes it possible to use a diffraction grating to analyze the spectral composition of radiation. Structure crystalline substance similar to three-dimensional grating. Observation of the diffraction pattern during the passage of X-rays, a beam of electrons or neurons through crystals in which particles of a substance (atoms, ions, molecules) are arranged in an orderly manner, makes it possible to study the features of their structure. The characteristic value for interatomic distances is d ~ 10 -10 m, which corresponds to the wavelengths of the radiation used and makes them indispensable for crystallographic analysis.

The diffraction of light determines the limit of the resolution of optical instruments (telescopes, microscopes, etc.). Resolution - the minimum distance between two objects at which they are seen separately, do not merge - are allowed. Due to diffraction, the image of a point source (for example, a star in a telescope) looks like a circle, so objects that are close together are not resolved. The resolution depends on a number of parameters, including the wavelength: the shorter the wavelength, the better the resolution. Therefore, the size of an object observed in an optical microscope is limited by the wavelength of light (approximately 0.5 µm).

The phenomenon of interference and diffraction of light underlies the principle of recording and reproducing images in holography. The method proposed in 1948 by D. Gabor (1900 - 1979) fixes the interference pattern obtained by illuminating an object and a photographic plate with coherent beams. The resulting hologram is an alternating light and dark spots that do not resemble the object, however, diffraction from the hologram of light waves identical to those used when recording it, allows you to restore the wave scattered by the real object and obtain its three-dimensional image.

Polarization- a phenomenon characteristic only of transverse waves. The transverseness of light waves (as well as any other electromagnetic waves) is expressed in the fact that the vectors of electric () and induction of magnetic () fields oscillating in them are perpendicular to the direction of wave propagation. In addition, these vectors are mutually perpendicular, so for complete description states of polarization of light, it is required to know the behavior of only one of them. The action of light on recording devices is determined by the intensity vector electric field, which is called the light vector.

light waves, emitted by a natural source of radiation i.e. set of independent atoms, are not polarized, because the direction of oscillation of the light vector () in a natural beam will change continuously and randomly, remaining perpendicular to the wave velocity vector.

Light in which the direction of the light vector remains unchanged is called linearly polarized. Polarization is the ordering of vector oscillations. An example is a harmonic wave. To polarize light, devices called polarizers are used, the operation of which is based on the features of the processes of reflection and refraction of light, as well as on the anisotropy of the optical properties of a substance in the crystalline state. The light vector in the beam passing through the polarizer oscillates in a plane called the plane of the polarizer. When polarized light passes through the second polarizer, it turns out that the intensity of the transmitted beam changes with the rotation of the polarizer. Light passes through the device without absorption if its polarization coincides with the plane of the second polarizer and is completely blocked by it when the crystal is rotated by 90 degrees, when the plane of oscillations of the polarized light is perpendicular to the plane of the second polarizer.

The polarization of light has found wide application in various industries scientific research and technology. it is used in microscopic studies, in the processes of sound recording, optical location, high-speed film and photography, in Food Industry(saccharimetry), etc.

Dispersion- dependence of the wave propagation velocity on their frequency (wavelength). When electromagnetic waves propagate in a medium, there arises -

The dispersion is determined physical properties the medium in which the waves propagate. For example, in a vacuum, electromagnetic waves propagate without dispersion, while in a real medium, even in such a rarefied one as the Earth's ionosphere, dispersion arises. Sonic and ultrasonic waves also detect dispersion. When they propagate in a medium, harmonic waves of different frequencies, into which a signal can be decomposed, propagate at different speeds, which leads to distortion of the signal shape. Dispersion of light - the dependence of the refractive index of a substance on the frequency (wavelength) of light. When the speed of light changes, depending on the frequency (wavelength), the refractive index changes. As a result of dispersion White light, consisting of many waves of different frequencies, when passing through a transparent trihedral prism, it decomposes and a continuous (continuous) spectrum is formed. The study of this spectrum led I. Newton (1672) to the discovery of the dispersion of light. For substances that are transparent in a given region of the spectrum, the refractive index increases with increasing frequency (decreasing wavelength), which corresponds to the distribution of colors in the spectrum. The highest refractive index is for violet light (=0.38 µm), the lowest for red (=0.76 µm). A similar phenomenon is observed in nature during the propagation sunlight in the atmosphere and its refraction in particles of water (summer) and ice (winter). This creates a rainbow or solar halo.

Doppler effect. The Doppler effect is a change in the frequency or wavelength perceived by the observer (receiver) due to the movement of the wave source and the observer relative to each other. Wave speed u is determined by the properties of the medium and does not change when the source or observer moves. If the observer or wave source moves at a speed relative to the medium, then the frequency v received waves becomes different. In this case, as established by K. Doppler (1803 - 1853), when the observer approaches the source, the frequency of the waves increases, and when removed, it decreases. This corresponds to a decrease in the wavelength λ as the source and observer approach each other and increase λ when they are mutually removed. For sound waves, the Doppler effect manifests itself in the increase in the pitch of the sound when the sound source and the observer approach each other (for 1 sec the observer perceives more waves), and correspondingly in a decrease in the tone of the sound when they are removed. The Doppler effect also causes the "redshift", as described above. - frequency reduction electromagnetic radiation from a moving source. This name is due to the fact that in the visible part of the spectrum, as a result of the Doppler effect, the lines are shifted to the red end; "Redshift" is also observed in radiation of any other frequencies, for example, in the radio range. The opposite effect associated with increasing frequencies is called blue (or violet) shift. In astrophysics, two "redshifts" are considered - cosmological and gravitational. Cosmological (metagalactic) refers to the "red shift" observed for all distant sources (galaxies, quasars) - a decrease in radiation frequencies, indicating the removal of these sources from each other and, in particular, from our Galaxy, i.e., about non-stationarity (expansion ) Metagalaxies. "Redshift" for galaxies was discovered by the American astronomer W. Slifer in 1912-14; in 1929, E. Hubble discovered that for distant galaxies it is greater than for nearby ones, and increases approximately in proportion to the distance. This made it possible to reveal the law of mutual removal (retreat) of galaxies. Hubble's law in this case is written in the form

u = HR; (3.14)

(u is the receding speed of the galaxy, r- distance to it, H - Hubble constant). Determining by the magnitude of the "redshift" the speed of removal of the galaxy, you can calculate the distance to it. To determine the distances to extragalactic objects using this formula, you need to know the numerical value of the Hubble constant N. The knowledge of this constant is also very important for cosmology: the definition of the "age" of the Universe is connected with it. In the early 1970s, the Hubble constant was taken to be H =(3 – 5)*10 -18 s -1 , reciprocal T = 1/H = 18 billion years. The gravitational "redshift" is a consequence of the slowing down of the pace of time and is due to the gravitational field (effect general theory relativity). This phenomenon is also called the Einstein effect or the generalized Doppler effect. It has been observed since 1919, first in the radiation of the Sun, and then in some other stars. In some cases (for example, during gravitational collapse) "redshift" of both types should be observed.

A pleasant picture can be observed in childhood: a quiet expanse of the surface of the water on the river. And one has only to throw a small pebble - this picture immediately changes. Around the place where the stone hit the water, waves run in circles. Everyone read stories about sea voyages, about the monstrous power of sea waves, easily rocking big ships. However, when observing these phenomena, not everyone knows that the sound of a splash of water reaches our ear in waves in the air that we breathe, that the light with which we visually perceive our surroundings is also a wave movement. Water waves, light and sound waves can be combined together. These are all examples of wave motion. But the waves have a different nature of appearance. What is a wave in terms of physics? A wave is an oscillation that propagates through space over time. The main property of waves is that the wave propagates without the transfer of matter. For example, if a small leaf from a tree lies on the surface of the water. Let's throw a stone into the water. From the stone in all directions, as mentioned earlier, waves will begin to spread. At the same time, having reached the leaf, they will not force it to move towards the wave. The leaf will remain in place, but at the same time it will make oscillatory movements up and down. That is, only the shape of the water will change, and there will be no current. One of the most important characteristics of water is the speed of its distribution. The propagation speed of any wave is always finite. The speed of the waves on the surface of the water is relatively small, so they are very easy to observe.
It is also easy to observe the waves propagating along the rubber cord. If one end of the cord is fixed and, slightly pulling the cord with your hand, bring the other end into an oscillatory motion, then a wave will run along the cord. The speed of the wave will be the greater, the stronger the cord is pulled. The wave will reach the point where the cord is fixed, will be reflected and run back. In this experiment, when the wave propagates, the shape of the cord changes. Each section of the cord oscillates about its unchanging equilibrium position. Let us pay attention to the fact that when the wave propagates along the cord, the oscillations occur in the direction perpendicular to the direction of wave propagation. Such waves are called transverse.
In this case, in such waves, an elastic deformation occurs, called shear deformation. Separate layers of matter are shifted relative to each other. When shear deformation occurs in a solid, elastic forces tend to return the body to its original state. It is the elastic forces that cause oscillations of the particles of the medium. But oscillations of particles of the medium can also occur along the direction of wave propagation. Such a wave is called longitudinal. It is convenient to observe the longitudinal wave on a long soft spring of large diameter. By hitting one of the ends of the spring with your palm, you can see how the compression (elastic impulse) runs along the spring. With the help of a series of successive impacts, it is possible to excite a wave in the spring, which is a successive compression and extension of the spring, running one after another.
Compressive deformation occurs in a longitudinal wave. The elastic forces associated with this deformation arise as in solids as well as in liquids and gases.
Acoustic waves can serve as examples of longitudinal waves, i.e. those that are perceived by the human ear. When a mechanical wave propagates, motion is transmitted from one particle of the medium to another. The transfer of motion is associated with the transfer of energy. The main property of all waves, regardless of their nature, is the transfer of energy without the transfer of matter. The energy comes from a source that excites vibrations at the beginning of the cord, string, etc., and propagates along with the wave. Energy is transmitted through any cross section, such as a cord. This energy is made up of kinetic energy motion of particles of the medium and potential energy their elastic deformation. The gradual decrease in the amplitude of particle oscillations during wave propagation is associated with the transformation of part of the mechanical energy into internal energy.
How does mechanical waves propagate? Let us follow the motion of individual particles of matter during wave motion. Let us first consider a transverse wave that propagates, for example, along a rubber cord. Each section of the cord has mass and elasticity. When the cord is deformed, elastic forces appear in any of its sections. These forces tend to return the cord to its original position. Due to inertia, a section of the oscillating cord does not stop at the equilibrium position, but passes through it, continuing to move until the elastic forces stop this section at the moment of maximum deviation from the equilibrium position. Instead of a cord, let's take a chain of identical metal balls suspended on threads. The balls are interconnected by springs (Fig.). The mass of the springs is much less than the mass of the balls. In this model, the inert (mass) and elastic properties are separated: the mass is concentrated mainly in the balls, and the elasticity is concentrated in the springs. This division is not essential when considering wave motion. If the left extreme ball is deflected in a horizontal plane perpendicular to the chain of balls, then the spring is deformed and a force will begin to act on the 2nd ball, forcing it to deviate in the same direction as the 1st ball. Due to inertia, the movement of the 2nd ball will not be consistent with the 1st. Its movement, repeating the movement of the 1st ball, will be delayed in time. If the 1st ball is forced to oscillate with a period T (just by hand or with the help of some mechanism), then the 2nd ball will also begin to oscillate after the 1st, but with some phase lag. The third ball, under the influence of the elastic force caused by the movement of the second ball, will also begin to oscillate, lagging behind in phase even more, and so on. Finally, all the balls will begin to perform forced oscillations with the same frequency, but with different phases. In this case, a transverse wave will run along the chain of balls. Figure a, b, c, d, e, f shows the process of wave propagation. The positions of the balls are shown at successive moments of time, spaced from each other by a quarter of the oscillation period (top view). The arrows of the balls are the velocity vectors of their movement at the corresponding moments in time. On the model of an elastic body in the form of a chain of massive balls connected by springs (Fig. a), one can observe the process of propagation of longitudinal waves. The balls are suspended so that they can only oscillate along the chain. If the 1st ball is brought into oscillatory motion with a period T, then a longitudinal wave will run along the chain, consisting of alternating compressions and rarefaction of the balls (Fig. b). This figure corresponds to figure e for the case of shear wave propagation.

>> Wave phenomena

§ 42 WAVE PHENOMENA

Each of us has observed how waves scatter in circles from a stone thrown onto the calm surface of a pond or lake (Fig. 6.1). Many watched the sea waves crashing on the shore. Everyone read stories about sea voyages, about the monstrous power of sea waves, easily rocking large ships. However, when observing these phenomena, not everyone knows that the sound of a splash of water reaches our ear in waves in the air that we breathe, that the light with which we visually perceive our surroundings is also a wave movement.

Wave processes are extremely widespread in nature. There are different physical causes that cause wave motions. But, like oscillations, all types of waves are described quantitatively by the same or almost the same laws. Many difficult-to-understand questions become clearer when comparing different wave phenomena.

What is called a wave? Why do waves occur? Separate particles of any body - solid, liquid or gaseous - interact with each other. Therefore, if any particle of the body begins to make oscillatory motions, then as a result of the interaction between the particles, this motion begins to spread in all directions with a certain speed.

A wave is an oscillation that propagates through space over time.

In air, solids and inside liquids, mechanical waves arise due to the action of elastic forces. These forces carry out the connection between the individual parts of the body. The formation of waves on the surface of water is caused by gravity and surface tension.

The main features of wave motion can be seen most clearly if we consider the waves on the surface of the water. It can be, for example, waves, which are rounded shafts running forward. The distances between the shafts, or ridges, are approximately the same. However, if a light object, such as a leaf from a tree, is on the surface of the water along which the wave is running, then it will not be carried forward by the wave, but will begin to oscillate up and down, remaining almost in one place.

When a wave is excited, the process of propagation of oscillations occurs, but not the transfer of matter. Vibrations of water that have arisen in some place, for example, from a thrown stone, are transmitted to neighboring areas and gradually spread in all directions, involving more and more particles of the medium in oscillatory movements. The flow of water does not arise, only local forms of its surface move.

Wave speed. The most important characteristic wave is the speed of its propagation. Waves of any nature do not propagate through space instantly. Their speed is finite. One can imagine, for example, that a gull flies over the sea, and in such a way that it always finds itself above the same crest of a wave. The speed of the wave in this case is equal to the speed of the seagull. Waves on the surface of the water are convenient for observation, since the speed of their propagation is relatively low.

Transverse and longitudinal waves. It is also easy to observe the waves propagating along the rubber cord. If one end of the cord is fixed and, slightly pulling the cord with your hand, bring its other end into oscillatory motion, then a wave will run along the cord (Fig. 6.2).

The speed of the wave will be the greater, the stronger the cord is pulled. The wave will reach the point where the cord is fixed, will be reflected and run back. In this experiment, when the wave propagates, the shape of the cord changes. Each section of the cord oscillates about its unchanging equilibrium position.

Let us pay attention to the fact that when the wave propagates along the cord, the oscillations occur in the direction perpendicular to the direction of wave propagation. Such waves are called transverse (Fig. 6.3). In a transverse wave, the displacements of individual sections of the medium occur in a direction perpendicular to the direction of wave propagation. In this case, an elastic deformation occurs, called shear deformation. Separate layers of matter are shifted relative to each other. When shear deformation occurs in a solid, elastic forces tend to return the body to its original state. It is the elastic forces that cause oscillations of the particles of the medium 1 .

The shift of layers relative to each other in gases and liquids does not lead to the appearance of elastic forces. Therefore, transverse waves cannot exist in gases and liquids. Transverse waves arise in solids.

But oscillations of the particles of the medium can also occur along the direction of wave propagation (Fig. 6.4). Such a wave is called longitudinal. It is convenient to observe the longitudinal wave on a long soft spring of large diameter. By hitting one of the ends of the spring with your palm (Fig. 6.5, a), you can see how the compression (elastic impulse) runs along the spring. With the help of a series of successive impacts, it is possible to excite a wave in the spring, which is a successive compression and extension of the spring, running one after another (Fig. 6.5, b).

So, in a longitudinal wave, compressive deformation occurs. The elastic forces associated with this deformation arise both in solids and in liquids and gases.

1 When we talk about the oscillations of the particles of the medium, we mean the oscillations of small volumes of the medium, and not the oscillations of the molecules.

These forces cause oscillations of individual sections of the medium. Therefore, longitudinal waves can propagate in all elastic media. In solids, the speed of longitudinal waves is greater than the speed of transverse waves.

This is taken into account when determining the distance from the earthquake source to the seismic station. First, a longitudinal wave is recorded at the station, since its velocity in the earth's crust is greater than that of the transverse wave. After some time, a transverse wave is recorded, which is excited during an earthquake simultaneously with the longitudinal one. Knowing the velocities of longitudinal and transverse waves in the earth's crust and the delay time of the transverse wave, it is possible to determine the distance to the earthquake source.

Wave energy. When a mechanical wave propagates, motion is transmitted from one particle of the medium to another. Related to the transfer of motion is the transfer of energy. The main property of all waves, regardless of their nature, is the transfer of energy without transferring the whole. The energy comes from a source that excites vibrations at the beginning of the cord, string, etc., and propagates along with the wave. Energy is transmitted through any cross section, such as a cord. This energy is composed of the kinetic energy of the motion of the particles of the medium and the potential energy of their elastic deformation. The gradual decrease in the amplitude of particle oscillations during wave propagation is associated with the transformation of part of the mechanical energy into internal energy.

A wave is an oscillation that propagates through space over time. Wave speed is finite. The wave transfers energy, but does not transfer the substance of the medium.

1. Which waves are called transverse and which are longitudinal!
2. Can a transverse wave propagate in water!

Myakishev G. Ya., Physics. Grade 11: textbook. for general education institutions: basic and profile. levels / G. Ya. Myakishev, B. V. Bukhovtsev, V. M. Charugin; ed. V. I. Nikolaev, N. A. Parfenteva. - 17th ed., revised. and additional - M.: Education, 2008. - 399 p.: ill.

Planning physics, materials on physics grade 11 download, textbooks online

These phenomena are inherent in waves of any nature. Moreover, the phenomena of interference, diffraction, and polarization are characteristic only of wave processes and can be explained only on the basis of wave theory.

Reflection and refraction. The propagation of waves is geometrically described using rays. In a homogeneous environment ( n= const) the rays are rectilinear. At the same time, at the interface between the media, their directions change. In this case, two waves are formed: reflected, propagating in the first medium with the same speed, and refracted, propagating in the second medium with a different speed, depending on the properties of this medium. The phenomenon of reflection is known for both sound (echo) and light waves. Due to the reflection of light, an imaginary image is formed in the mirror. The refraction of light underlies many interesting atmospheric phenomena. It is widely used in various optical devices: lenses, prisms, optical fibers. These devices are elements of devices for various purposes: cameras, microscopes and telescopes, periscopes, projectors, optical communication systems, etc.

Interference waves - the phenomenon of energy redistribution when two (or several) coherent (matched) waves are superimposed, accompanied by the appearance of an interference pattern of alternating maxima and minima of the intensity (amplitude) of the resulting wave. Waves are called coherent, for which the phase difference at the point of addition remains unchanged in time, but can change from point to point and in space. If the waves meet ʼʼin phaseʼʼ, ᴛ.ᴇ. simultaneously reach the maximum deviation in one direction, then they reinforce each other, and if they meet ʼʼin antiphaseʼʼ, ᴛ.ᴇ. simultaneously achieve opposite deviations, then weaken each other. Coordination of the oscillations of two waves (coherence) of two waves in the case of light is possible only if they have a common origin, which is due to the peculiarities of the radiation processes. The exception is lasers, whose radiation is characterized by high coherence. For this reason, to observe interference, light coming from one source is divided into two groups of waves, either passing through two holes (slits) in an opaque screen, or due to reflection and refraction at the interface in thin films. Interference pattern from a monochromatic source ( λ=const) on the screen for rays passing through two narrow closely spaced slits, has the form of alternating bright and dark stripes (Jung's experiment, 1801 ᴦ.). Bright stripes - intensity maxima are observed at those points of the screen where the waves from two slits meet ʼʼin phaseʼʼ, i.e. their phase difference

, m =0,1,2,…,(3.10)

This corresponds to the difference in the path of the rays, a multiple of an integer number of wavelengths λ

, m =0,1,2,…,(3.11)

Dark stripes (mutual repayments), ᴛ.ᴇ. intensity minima occur at those points of the screen where the waves meet ʼʼin antiphaseʼʼ, i.e. their phase difference is

, m =0,1,2,…,(3.12)

This corresponds to the difference in the path of the rays, a multiple of an odd number of half-waves

, m =0,1,2,….(3.13)

Interference is observed for different waves. White light interference, including all wavelengths of visible light in the wavelength range microns can appear as iridescent coloration of thin films of gasoline on the surface of water, soap bubbles, oxide films on the surface of metals. The conditions of the interference maximum at different points of the film are satisfied for different waves with different wavelengths, which leads to amplification of waves of different colors. The interference conditions are determined by the wavelength, which for visible light is a fraction of a micron (1 μm = 10 -6 m), in this regard, this phenomenon underlies various precision (ʼʼultrapreciseʼʼ) methods of research, control and measurement. The use of interference is based on the use of interferometers, interference spectroscopes, as well as the holography method. Light interference is used to measure the wavelength of radiation, study the fine structure of spectral lines, determine densities, refractive indices of substances, and thicknesses of thin coatings.

Diffraction- a set of phenomena that occur during the propagation of a wave in a medium with a pronounced inhomogeneity of properties. This is observed when waves pass through a hole in the screen, near the border of opaque objects, etc. Diffraction causes the wave to wrap around an obstacle whose dimensions are commensurate with the wavelength. If the size of the obstacle is much larger than the wavelength, then the diffraction is weakly manifested. On macroscopic obstacles, diffraction of sound, seismic waves, radio waves is observed, for which 1 cm km. It is worth saying that in order to observe the diffraction of light, the obstacles must have significantly smaller dimensions. The diffraction of sound waves explains the ability to hear the voice of a person who is around the corner of the house. The diffraction of radio waves around the Earth's surface explains the reception of radio signals in the range of long and medium radio waves far beyond the line of sight of the emitting antenna.

The diffraction of waves is accompanied by their interference, which leads to the formation of a diffraction pattern, alternating intensity maxima and minima. When light passes through a diffraction grating, which is a set of alternating parallel transparent and opaque bands (up to 1000 per 1 mm), a diffraction pattern appears on the screen, the position of the maxima of which depends on the radiation wavelength. This makes it possible to use a diffraction grating to analyze the spectral composition of radiation. The structure of a crystalline substance is similar to a three-dimensional diffraction grating. Observation of the diffraction pattern during the passage of X-rays, a beam of electrons or neurons through crystals in which particles of a substance (atoms, ions, molecules) are arranged in an orderly manner, makes it possible to study the features of their structure. The characteristic value for interatomic distances is d ~ 10 -10 m, which corresponds to the wavelengths of the radiation used and makes them indispensable for crystallographic analysis.

The diffraction of light determines the limit of the resolution of optical instruments (telescopes, microscopes, etc.). Resolution - the minimum distance between two objects at which they are seen separately, do not merge - are allowed. Due to diffraction, the image of a point source (for example, a star in a telescope) looks like a circle, so objects that are close together are not resolved. The resolution depends on a number of parameters, including the wavelength: the shorter the wavelength, the better the resolution. For this reason, the size of an object observed in an optical microscope is limited by the wavelength of light (approximately 0.5 µm).

The phenomenon of interference and diffraction of light underlies the principle of recording and reproducing images in holography. The method proposed in 1948 by D. Gabor (1900 - 1979) fixes the interference pattern obtained by illuminating an object and a photographic plate with coherent beams. The resulting hologram is an alternating light and dark spots that do not resemble the object, however, diffraction from the hologram of light waves identical to those used when recording it, makes it possible to restore the wave scattered by the real object and obtain its three-dimensional image.

Polarization- a phenomenon characteristic only of transverse waves. The transverseness of light waves (as well as any other electromagnetic waves) is expressed in the fact that the vectors of electric () and magnetic induction () fields oscillating in them are perpendicular to the direction of wave propagation. At the same time, these vectors are mutually perpendicular; therefore, to fully describe the state of light polarization, it is required to know the behavior of only one of them. The action of light on the recording devices is determined by the electric field strength vector, which is called the light vector.

Light waves emitted by a natural radiation source ᴛ.ᴇ. set of independent atoms, are not polarized, because the direction of oscillation of the light vector () in a natural beam will change continuously and randomly, remaining perpendicular to the wave velocity vector.

Light in which the direction of the light vector remains unchanged is called linearly polarized. Polarization is the ordering of vector oscillations. An example is a harmonic wave. To polarize light, devices called polarizers are used, the operation of which is based on the features of the processes of reflection and refraction of light, as well as on the anisotropy of the optical properties of a substance in the crystalline state. The light vector in the beam passing through the polarizer oscillates in a plane called the plane of the polarizer. When polarized light passes through the second polarizer, it turns out that the intensity of the transmitted beam changes with the rotation of the polarizer. Light passes through the device without absorption if its polarization coincides with the plane of the second polarizer and is completely blocked by it when the crystal is rotated by 90 degrees, when the plane of oscillations of the polarized light is perpendicular to the plane of the second polarizer.

The polarization of light has found wide application in various branches of scientific research and technology. it is used in microscopic research, in sound recording, optical location, high-speed film and photography, in the food industry (saccharimetry), etc.

Dispersion- dependence of the wave propagation velocity on their frequency (wavelength). When electromagnetic waves propagate in a medium, there arises -

The dispersion is determined by the physical properties of the medium in which the waves propagate. For example, in a vacuum, electromagnetic waves propagate without dispersion, while in a real medium, even in such a rarefied one as the Earth's ionosphere, dispersion arises. Sonic and ultrasonic waves also detect dispersion. When they propagate in a medium, harmonic waves of different frequencies, into which the signal should be decomposed, propagate at different speeds, which leads to distortion of the signal shape. Dispersion of light - the dependence of the refractive index of a substance on the frequency (wavelength) of light. When the speed of light changes based on the frequency (wavelength), the refractive index changes. As a result of dispersion, white light, consisting of many waves of different frequencies, decomposes when passing through a transparent trihedral prism and forms a continuous (continuous) spectrum.
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The study of this spectrum led I. Newton (1672) to the discovery of the dispersion of light. For substances that are transparent in a given region of the spectrum, the refractive index increases with increasing frequency (decreasing wavelength), which corresponds to the distribution of colors in the spectrum. The highest refractive index is for violet light (=0.38 µm), the lowest for red (=0.76 µm). A similar phenomenon is observed in nature during the propagation of sunlight in the atmosphere and its refraction in particles of water (in summer) and ice (in winter). This creates a rainbow or solar halo.

Doppler effect. The Doppler effect is a change in the frequency or wavelength perceived by the observer (receiver) due to the movement of the wave source and the observer relative to each other. Wave speed u is determined by the properties of the medium and does not change when the source or observer moves. If the observer or wave source moves at a speed relative to the medium, then the frequency v received waves becomes different. In this case, as established by K. Doppler (1803 - 1853), when the observer approaches the source, the frequency of the waves increases, and when removed, it decreases. This corresponds to a decrease in the wavelength λ as the source and observer approach each other and increase λ when they are mutually removed. For sound waves, the Doppler effect manifests itself in the increase in the pitch of the sound when the sound source and the observer approach each other (for 1 sec the observer perceives a larger number of waves), and correspondingly in the lowering of the tone of the sound when they are removed. The Doppler effect also causes the ʼʼredshiftʼʼ, as described above. - lowering the frequencies of electromagnetic radiation from a moving source. This name is due to the fact that in the visible part of the spectrum, as a result of the Doppler effect, the lines are shifted to the red end; ʼʼredshiftʼʼ is also observed in the radiation of any other frequencies, for example, in the radio range. The opposite effect associated with an increase in frequencies is commonly called blue (or violet) shift. In astrophysics, two ʼʼredshiftsʼʼ are considered - cosmological and gravitational. Cosmological (metagalactic) is called ʼʼredshiftʼʼ, observed for all distant sources (galaxies, quasars) - a decrease in radiation frequencies, indicating the removal of these sources from each other and, in particular, from our Galaxy, i.e. about non-stationarity (expansion ) Metagalaxies. ʼʼRedshiftʼʼ for galaxies was discovered by the American astronomer W. Slifer in 1912-14; in 1929, E. Hubble discovered that for distant galaxies it is greater than for nearby ones, and increases approximately in proportion to the distance. This made it possible to reveal the law of mutual removal (retreat) of galaxies. Hubble's law in this case is written in the form

u = HR; (3.14)

(u is the receding speed of the galaxy, r- distance to it, H - Hubble constant). Determining by the magnitude of the ʼʼʼʼʼʼʼʼe ʼʼremoval velocity of the galaxy, one can calculate the distance to it. To determine the distances to extragalactic objects using this formula, you need to know the numerical value of the Hubble constant N. The knowledge of this constant is also very important for cosmology: the definition of the ʼʼageʼʼ of the Universe is connected with it. In the early 1970s, the Hubble constant was taken to be H =(3 – 5)*10 -18 s -1 , reciprocal T = 1/H = 18 billion years. Gravitational ʼʼredshiftʼʼ is a consequence of slowing down the pace of time and is due to the gravitational field (the effect of the general theory of relativity). This phenomenon is also called the Einstein effect or the generalized Doppler effect. It has been observed since 1919, first in the radiation of the Sun, and then in some other stars. In a number of cases (for example, during gravitational collapse), a "redshift" of both types should be observed.

Physical nature of wavesMechanical
elastic
On a surface
liquids
electromagnetic
light
x-ray
Sound
radio waves
seismic

A mechanical wave is an oscillation of particles of matter propagating in space.

The points of the medium in which waves oscillating in one phase propagate are called wave surfaces.

Two conditions are necessary for the occurrence of a mechanical wave:

Comparing the direction of wave propagation and the direction of oscillation of the points of the medium, it is possible to distinguish between longitudinal and transverse waves.

Waves in which the direction of oscillation of the points of the excited medium is parallel to the direction of wave propagation are called longitudinal.

Waves in which the direction of oscillation of the points of the excited medium is perpendicular to the direction of wave propagation are called transverse

Waves on the surface of a liquid are neither longitudinal nor transverse. Thus, a wave on the surface of a liquid is

Circular waves on the surface of a liquid

Properties of mechanical waves

If a wave passes from one medium to another, falling on the interface between two media at some angle other than zero, then it experiences

A wave can go around obstacles whose dimensions are commensurate with its length. The phenomenon of waves bending around obstacles is called diffraction.

Wave sources oscillating with the same frequency and constant phase difference are called coherent. Like any wave formed by

Sound is elastic waves that propagate in gases, liquids, solids and are perceived by the human and animal ears. mechanical waves

To hear the sound, you need:

Mechanical waves arising in elastic media in which the particles of the medium oscillate with frequencies lower than the frequencies of the sound range

Mechanical waves arising in elastic media, in which the particles of the medium oscillate with frequencies greater than the frequencies of the sound range