We now turn to the study of the propagation of oscillations. If we are talking about mechanical vibrations, that is, about the oscillatory motion of particles of any solid, liquid or gaseous medium, then the propagation of vibrations means the transmission of vibrations from one particle of the medium to another. The transmission of oscillations is due to the fact that adjacent sections of the medium are interconnected. This connection can be carried out in various ways. It can be caused, in particular, by the elastic forces arising from the deformation of the medium during its vibrations. As a result, a vibration caused in any way in one place entails the successive occurrence of vibrations in other places, more and more distant from the original, and a so-called wave arises.

Mechanical wave phenomena are of great importance for everyday life. These phenomena include the propagation of sound vibrations, due to the elasticity of the air around us. Thanks to elastic waves, we can hear at a distance. Circles running up on the surface of the water from a thrown stone, small ripples on the surface of the lake and huge ocean waves are also mechanical waves, although of a different type. Here the connection of adjacent sections of the water surface is due not to the force of elasticity, but to the force of gravity (§ 38) or the forces of surface tension (see Volume I, § 250). In the air, not only sound waves can propagate, but also destructive blast waves from exploding shells and bombs. Seismic stations record ground vibrations caused by earthquakes occurring thousands of kilometers away. This is possible only because seismic waves propagate from the place of the earthquake - vibrations in the earth's crust.

A huge role is also played by wave phenomena of a completely different nature, namely electromagnetic waves. These waves represent the transmission from one place in space to another of the oscillations of the electric and magnetic fields created by electric charges and currents. The connection between neighboring sections of the electromagnetic field is due to the fact that any change in the electric field causes the appearance of a magnetic field, and vice versa, any change in the magnetic field creates an electric field (§ 54), A solid, liquid or gaseous medium can greatly affect the propagation of electromagnetic waves, but the presence such a medium is not necessary for these waves. Electromagnetic waves can propagate wherever an electromagnetic field can exist, and hence in a vacuum, i.e., in a space that does not contain atoms.

The phenomena caused by electromagnetic waves include, for example, light. Just as a certain frequency range of mechanical vibrations is perceived by our ear and gives us the sensation of sound, so a certain (and, as we shall see, very narrow) frequency range of electromagnetic vibrations is perceived by our eye and gives us the sensation of light.

By observing the propagation of light, one can directly verify that electromagnetic waves can propagate in a vacuum. By placing an electric or clockwork bell under the glass bell of an air pump and pumping out the air, we find that the sound gradually fades away as it is pumped out and finally stops. The picture of everything that is under the bell and behind it, visible to the eye, does not experience any changes. It is difficult to overestimate this property of electromagnetic waves. Mechanical waves do not go beyond the earth's atmosphere; electromagnetic waves open to us the widest expanses of the universe. Light waves allow us to see the Sun, stars and other celestial bodies, separated from us by huge "empty" spaces; With the help of electromagnetic waves of very different lengths that reach us from these distant bodies, we can draw the most important conclusions about the structure of the Universe.

In 1895 Russian physicist and inventor Alexander Stepanovich Popov (1859-1906) discovered a new boundless field of application of electromagnetic waves. He invented equipment that makes it possible to use these waves for signal transmission - telegraphy without wires. Thus was born wireless communication, or radio, thanks to which a vast range of electromagnetic waves, much longer than light waves, received exceptional practical and scientific significance (§ 60).

The present development of this greatest invention is such that one can justifiably speak of the radio as one of the marvels of modern technology. Nowadays, radio makes it possible not only to carry out wireless telegraph and telephone communications between any points on the globe, but also to transmit images (television and phototelegraphy), control machines and projectiles at a distance (telecontrol), detect and even see distant objects that themselves do not emit radio waves by themselves (radar), drive ships and aircraft along a given course (radio navigation), observe the radio emission of celestial bodies (radio astronomy), etc.

Below we will consider some of the applications of electromagnetic waves mentioned here in more detail. But even a simple (and far from complete) enumeration of these applications says a lot about the exceptional significance of these waves.

Despite the different nature of mechanical and electromagnetic waves, there are many general patterns inherent in any wave phenomena. One of the main laws of this kind is that any wave propagates from one point to another not instantly, but with a certain speed.

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: a reflected one, propagating in the first medium with the same speed, and a refracted one, 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 redistribution of energy 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 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 at which 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 visible light waves 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 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 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 close objects 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, 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 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 usually 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.
Hosted on ref.rf
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 value of the ʼʼʼʼʼʼʼ the removal rate 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 associated 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.

Municipal budgetary educational institution - secondary

Secondary school No. 2 named after A.I. Herzen, Klintsy, Bryansk region

Lesson on the topic

Prepared and hosted:

Physics teacher

Prokhorenko Anna

Alexandrovna

Klintsy, 2013

Content:

Lesson on the topic “Wave phenomenon. Propagation of mechanical waves. Wavelength. Wave speed. »

The purpose of the lesson: introduce the concepts of wave, wave length and speed, wave propagation condition, types of waves, teach students to apply formulas for finding the length and speed of a wave; to study the causes of the propagation of transverse and longitudinal waves;

Methodical tasks:

Educational : familiarization of students with the origin of the term "wave, wavelength, wave speed"; show students the phenomenon of wave propagation, and also prove, with the help of experiments, the propagation of two types of waves: transverse and longitudinal.

Educational : to promote the development of speech, thinking, cognitive and general labor skills; to promote mastering the methods of scientific research: analysis and synthesis.

Educational :

Lesson type: learning new material.

Methods: verbal, visual, practical.

Equipment: computer, presentation.

Demos:

Transverse and longitudinal waves.

Propagation of transverse and longitudinal waves.

Lesson plan:

Organization of the beginning of the lesson.

motivational stage. Setting goals, objectives of the lesson.

Learning new material

Consolidation of new knowledge.

Summing up the lesson.

DURING THE CLASSES

1. Organizational stage

2. motivational stage. Setting goals, objectives of the lesson.

What did you see in these videos? (Waves)

What types of waves did you see?

Based on your answers, we will try to set goals for today's lesson with you, for this, let's remember what is the plan for studying the concept, in this case, the concept of a wave? (What is a wave, i.e. definition, types of waves, characteristics of waves)

In today's lesson, I will help you with the concepts of wave, wave length and speed, wave propagation condition, types of waves, teach students to apply formulas for finding the length and speed of a wave; to study the causes of the propagation of transverse and longitudinal waves;with to form a conscientious attitude to educational work, positive motivation for learning, communication skills; contribute to the education of humanity, discipline, aesthetic perception of the world.

1. Learning new material

Now you need, according to the plan, which is presented on the screen and on the sheets of paper on your desks, and after reading paragraphs 42 and 43, find the necessary information and write it out.

Plan:

Wave concept

Conditions for the occurrence of a wave

Wave source

What is needed for a wave to occur

Types of waves (definitions)

Wave - vibrations that propagate in space over time. Waves arise mainly due to elastic forces.

Wave features:

Mechanical waves can propagate only in some medium (substance): in a gas, in a liquid, in a solid.

A mechanical wave cannot arise in a vacuum.

The source of the waves are oscillating bodies that create a deformation of the medium in the surrounding space. (rice)

For the occurrence of a mechanical wave, it is necessary:

1. The presence of an elastic medium

2 . The presence of a source of vibrations - deformation of the medium

Wave types:

Transverse - in which oscillations occur perpendicular to the direction of wave movement. Occur only in solids.

Longitudinal- in which oscillations occur along the direction of wave propagation.Occur in any medium (liquids, gases, solids).

We consider a table summarizing previous knowledge. (Look at the presentation)

We conclude: mechanical wave:

the process of vibration propagation in an elastic medium;

in this case, energy is transferred from particle to particle;

there is no transfer of matter;

To create a mechanical wave, an elastic medium is needed: liquid, solid or gas.

And now we will consider and write down the main characteristics of the waves.

What quantities characterize the wave

Each wave propagates at a certain speed. Under speedvwaves understand the propagation velocity of the perturbation. The speed of a wave is determined by the properties of the medium in which this wave propagates. When a wave passes from one medium to another, its speed changes.

The wavelength λ is the distance over which the wave propagates in a time equal to the period of oscillations in it.

Main characteristics: λ=v* T, λ - wavelength m,vis the propagation velocity m/s, T is the wave period c.

4. Consolidation of new knowledge.

What is a wave?

Wave conditions?

What types of waves do you know?

Can a transverse wave propagate in water?

What is called wavelength?

What is the speed of wave propagation?

How to relate speed and wavelength?

We consider 2 types and determine where which wave is?

Solve problems:

Determine the wavelength at a frequency of 200 Hz if the wave propagation speed is 340 m/s. (68000 m=68 km)

On the surface of the water in the lake, the wave propagates at a speed of 6 m/s. A leaf of a tree floats on the surface of the water. Determine the frequency and period of oscillation of the leaf if the wavelength is 3 m. (0.5 m, 2 s -1 )

The wavelength is 2 m, and the speed of its propagation is 400 m/s. Determine how many complete oscillations this wave makes in 0.1 s (20)

We consider it interesting : Waves on the surface of a liquid are neither longitudinal nor transverse. If you throw a small ball on the surface of the water, you can see that it moves, swaying on the waves, along a circular path. Thus, a wave on the surface of a liquid is the result of the addition of the longitudinal and transverse motion of water particles.

5. Summing up the lesson.

So, let's sum up.

What words would you use to describe the state after the lesson?:

Knowledge is knowledge only when it is acquired by the efforts of one's thought, and not by memory;

Oh, how tired I am from this fuss ... ..

You understood the bliss of studies, luck, law and secret

Studying the topic "Mechanical waves" is not so easy!!!

6 . Information about homework.

Schedule responses to questions using §§42-44

It is good to know the formulas and definitions on the topic "Waves"

Optional: make a crossword on the topic "Mechanical waves"

Tasks:

The fisherman noticed that in 10 seconds the float made 20 oscillations on the waves, and the distance between adjacent wave humps was 1.2 m. What is the speed of wave propagation?(T=n/t; T=10/5=2c; λ=υ*ν; ν=1/T; λ=υ/T; υ=λ*T*υ=1*2=2(m/s ))

The length of the wave is 5 m, and its frequency is 3 Hz. Determine the speed of the wave. (1.6 m/s)

Introspection

The lesson was held in grade 11 on the topic “wave phenomenon. Propagation of mechanical waves. Wavelength. Wave speed.It is the thirteenth lesson in the physics section "Mechanical oscillations and waves." Type of lesson: learning new material.

The lesson took into account the triune didactic goal: educational, developing, upbringing. I set the educational goal to familiarize students with the origin of the term "wave, wavelength, wave speed"; show students the phenomenon of wave propagation, and also prove with the help of experiments the existence of two types of waves: transverse and longitudinal. As a developing goal, I set the formation of students' clear ideas about the conditions for wave propagation; development of logical and theoretical thinking, imagination, memory in solving problems and consolidating ZUNs. I have set as an educational goal: to form a conscientious attitude to educational work, positive motivation for learning, communication skills; contribute to the education of humanity, discipline, aesthetic perception of the world.

During the lesson, we went through the following steps:

Organizational stage

Motivational and goal setting, lesson objectives. At this stage, based on the watched video clip, we determined the goals and objectives for the lesson and conducted motivation. Using: a verbal method in the form of a conversation, a visual method in the form of watching a video clip.

Learning new material

At this stage, I provided for a logical connection when explaining new material: consistency, accessibility, understandability. The main methods of the lesson were: verbal (conversation), visual (demonstrations, computer modeling). Form of work: individual.

Fixing new material

When fixing the students' ZUNs, I used interactive tasks from the multimedia manual in the "Mechanical Waves" section, solving problems at the blackboard with an explanation. The main methods of the lesson were: practical (problem solving), verbal (talk on questions)

Summarizing.

At this stage, I used the verbal method in the form of a conversation, the guys answered the questions posed.

Reflection done. We found out whether the goals set at the beginning of the lesson were achieved, which was difficult for them in this lesson. Two students were given marks for the tasks and several students were given marks for the answers.

Information about homework.

At this stage, students were asked to write down their homework in the form of an answer to a question according to the plan and a couple of tasks on a piece of paper. And optionally make a crossword puzzle.

I believe that the triune didactic goal of the lesson has been achieved.

Physical nature of wavesMechanical
elastic
On the 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:

The presence of the environment.
The presence of a source of vibrations.

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 in which direction
fluctuations of the points of the excited medium
perpendicular to the direction
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

Waves on
surfaces
liquids are not
are neither
longitudinal, nor
transverse. So
way, wave on
surfaces
liquids
represents
superposition
longitudinal and
transverse
molecular movements.

Circular waves on the surface of a liquid

Observation of waves on the surface of a liquid
allows you to explore and visualize many
wave phenomena common to different types of waves:
interference, diffraction, wave reflection, etc.

Properties of mechanical waves

All waves reaching the interface
two media experience reflection

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

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

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

Wave sources oscillating with the same
frequency and constant phase difference
are called coherent.
Like any waves formed by coherent
sources may overlap, and
as a result of superposition, there is
wave interference.

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

Sound is elastic waves
propagating in gases, liquids,
solid bodies and perceived by the ear
man and animals.
Mechanical waves that cause
the sensation of sound is called sound
waves.

sound waves
represent
longitudinal waves,
which is happening
alternation of condensations and
discharges.

To hear the sound, you need:

sound source;
elastic medium between it and the ear
certain range of vibration frequencies
sound source - between 16 Hz and 20000 Hz;
sufficient for ear perception
sound wave power.

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 generated
in elastic media, in which
the particles of the medium oscillate with
frequencies lower than the frequencies
audio range are called
infrasonic waves.

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

mechanical waves,
emerging in
elastic media,
which particles
environments fluctuate with
frequencies, large
than the frequencies of the sound
range are called
ultrasonic
waves.

>> 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 of a 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 filament, 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.

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