The set of frequencies of electromagnetic waves that are present in the radiation of any body is called emission spectrum.

Spectra are solid, ruled and striped.

Continuous Spectra give all substances that are in solid or liquid state. The continuous spectrum contains waves of all frequencies of visible light and therefore looks like a colored band with a smooth transition from one color to another in this order: red, orange, yellow, green, blue, indigo and violet ("Every hunter wants to know where the pheasant sits" ).

Line Spectra give all substances in the gaseous atomic state. Isolated atoms of all substances radiate sets of waves of quite definite frequencies peculiar only to them. As each person has his own personal fingerprints, so the atom of a given substance has its own spectrum, characteristic only for it. Line emission spectra look like colored lines separated by gaps. The nature of line spectra is explained by the fact that the atoms of a particular substance have only their own stationary states with their own characteristic energy, and, consequently, their own set of pairs of energy levels that an atom can change, i.e., an electron in an atom can only transfer from one specific orbits to other, well-defined orbits for a given chemical.

Striped Spectra are created by molecules that are not bound or weakly bound to each other. Striped spectra look like line spectra, only instead of individual lines, separate series of lines are observed, perceived as separate bands separated by dark gaps.

It is characteristic that what spectrum is emitted by these atoms, the same is absorbed, i.e. the emission spectra in terms of the set of emitted frequencies coincide with the absorption spectra. Since atoms of different substances correspond to spectra peculiar only to them, there is a way to determine the chemical composition of a substance by studying its spectra. This method is called spectral analysis. Spectral analysis is used to determine the chemical composition of mineral ores during mining, to determine the chemical composition of planetary atmospheres; is the main method for monitoring the composition of a substance in metallurgy and mechanical engineering.

The flame emits light. Glass absorbs ultraviolet rays. Ordinary phrases, familiar concepts. However, here the terms "radiate", "absorb" describe only externally, easily observing, the physics of these processes is directly related to the structure of atoms and molecules of matter.

An atom is a quantum system, its internal energy is basically the energy of the interaction of electrons with the nucleus; this energy, according to quantum laws, can only have values ​​that are quite definite for the quantum and the state of atoms. Thus, the energy of an atom cannot change continuously, but only in jumps - in portions equal to the difference of any two allowed energy values.

A quantum system (atom, molecule), receiving a portion of energy from outside, is excited, i.e. moves from one energy level to another higher one. The system cannot stay in an excited state for an arbitrarily long time; at some point, a spontaneous (spontaneous) reverse transition occurs with the release of the same energy. Quantum transitions can be radiative and non-radiative. In the first case, energy is absorbed or emitted in the form of a portion of electromagnetic radiation, the frequency of which is strictly determined by the energy difference between the levels between which the transition occurs. In the case of nonradiative transitions, the system receives or gives off energy when interacting with other systems (atoms, molecules, electrons). The presence of these two types of transitions is explained by optoacoustic Beinger effect.

When a gas in a closed volume is irradiated, modulated by an infrared radiation flux, pressure pulsations occur in the gas (about ptico-acoustic effect). Its mechanism is quite simple; absorption of infrared radiation occurs with the excitation of gas molecules, while the reverse transition occurs nonradiatively, i.e. the excitation energy of the molecules is converted into their kinetic energy, which causes a change in pressure.

The quantitative characteristics of the effect are very sensitive to the composition of the gas mixture. The use of the optical-acoustic effect for analysis is characterized by simplicity and reliability, high selectivity and a wide range of component concentrations.

The opto-acoustic indicator is a non-selective radiant energy receiver designed for gas analysis. The modulated radiant flux through the fluorite window enters the chamber with the gas under study. Under the action of the flow, the gas pressure on the microphone membrane changes, as a result of which electrical signals appear in the microphone circuit, depending on the composition of the gas.

The opto-acoustic effect is used in measuring the lifetime of excitation of molecules, in a number of works on the determination of humidity and radiation fluxes. Note that the optical-acoustic effect is also possible in liquids and solids.

The atoms of each substance have their own energy level structure inherent only to them, and, consequently, the structure of impulse transitions that can be registered by optical methods (for example, photographically). This circumstance underlies spectral analysis. Since molecules are also purely quantum systems, each substance (a collection of atoms or molecules) emits and absorbs only quanta of certain energies or electromagnetic radiation of certain wavelengths). The intensity of certain spectral lines is proportional to the number of atoms (molecules) that emit (or absorb) light. This ratio forms the basis of quantitative spectral analysis.

Application example of spectral analysis:

The concentration of known gases in the mixture is measured by the transmission of radiation from a laser source with a certain wavelength. Each of the gases contained in the mixture, the concentration of which is known, is preliminarily irradiated with monochromatic radiations with different wavelengths, and the absorption coefficient of each gas is determined for each wavelength. Then, at these wavelengths, the absorption of the test mixture is changed and, using the obtained absorption coefficient values, the concentration of each gas in the mixture is determined. When measuring with radiation containing more wavelengths than there are components in the gas mixture, the presence of unknown gases can be detected.

For atoms and molecules, the emission spectra will be line and striped, respectively, and the same for absorption spectra. To obtain a continuous spectrum, the presence of a plasma is necessary, i.e. ionized state of matter. During ionization, electrons are outside the atom or molecule, and, therefore, can have any continuously changing energy. When these electrons and ions are recommended, a continuous spectrum is obtained in which all wavelengths are present.

Excitation(increase in internal energy) or ionization of atoms occur under the influence of various causes; in particular, the energy for these processes can be obtained by heating bodies. The higher the temperature, the greater the excitation energy and the shorter and shorter waves (quanta with higher energy) the heated body radiates. Therefore, with gradual heating, infrared radiation (long waves) first appears, then red, to which orange, yellow, etc. are added with increasing temperature; eventually receives light. Further heating leads to the appearance of an ultraviolet component.

Application examples:

A device for continuous measurement of the temperature of a liquid metal bath contains a rod made of a translucent material with high temperature and corrosion resistance. The rod passes through the wall of the tank and inside the latter is embedded in a mass of alkali-free oxide with a high melting point, such as zirconium oxide. The end of the rod, located in the tank, serves as a color pyrometer.

Radiative and non-radiative transitions in infrared. areas are often used for processes and cooling:

A glass-forming tool comprising a coated metal body, characterized in that, for the purpose of completeness and improvement of the quality of products, the coating is made in two layers, the intermediate layer being made of a material that absorbs the near infrared region, for example, graphite, and the outer layer is made of a material that is transparent in the skin. spectral regions, for example based on transparent polycrystalline alumina;

A method for measuring the thermal conductivity of solids, including isothermal exposure to its cooling at a constant ambient temperature and registration of temperature changes, characterized in that, in order to measure partially transparent materials, the sample at the absorption stage is placed in a vacuum space and the energy emitted by the surface of the sample in the spectral range is measured. areas of strong absorption.

Radiative quantum transitions can occur not only spontaneously, but also forced under the action of external radiation, the frequency of which is consistent with the energy of this transition. The emission of light quanta by atoms and molecules of a substance under the action of an external electromagnetic field (radiation) is called forced or induced emission.

An essential difference between stimulated emission is that it is an exact copy of the forcing emission. All characteristics coincide - frequency, polarization, direction of propagation and phase. Because of this, stimulated emission can, under certain circumstances, lead to amplification of the external radiation that has passed through the substance, instead of its absorption. Therefore, otherwise stimulated emission is called negative absorption.

For the occurrence of stimulated emission, the presence of excited atoms in the substance is necessary, i.e. atoms at higher energy levels. Usually the fraction of such atoms is small. In order to amplify the radiation passing through it, it is necessary that the fraction of excited atoms be large, so that the levels with higher energy are "populated" with particles more densely than the lower levels. This state of matter is called state with population inversion.

Discovery by Soviet physicists Fabrikant, Vudynsky and Butaeva phenomena of amplification of electromagnetic waves when passing through a medium with population inversion was fundamental in the development optical quantum generators (lasers) the biggest invention of the century.

A rod of matter with an artificially created population inversion, placed between two mirrors, one of which is translucent - this is the schematic diagram of the simplest laser.

An optical resonator of two mirrors is needed to create feedback: part of the radiation returns to the working body, inducing a new avalanche of photons. Laser radiation is monochromatic and coherent due to the properties of stimulated radiation.

The fields of application of lasers are determined by the main characteristics of their radiation, such as coherence, monochromaticity, high energy concentration in the beam and its low divergence. In addition to the already traditional fields of application of lasers, such as the processing of superhard and refractory materials, laser communication and loya medicine and the production of high-temperature plasma, new interesting areas of their use began to be identified.

Recently developed dye lasers are extremely promising, unlike conventional ones, which allow smoothly changing the radiation frequency in a wide range from infrared to ultraviolet. So, for example, it is supposed to break with a laser beam, or vice versa, to create strictly defined bonds.

Work is underway to separate isotopes using tunable lasers. By changing the frequency of lasers, they tune it into resonance with a certain quantum transition of one of the isotopes and thereby transfer the isotope to an excited state in which it can be ionized and, using electrical reactions, separated from other isotopes.

And here is a purely inventive use of a laser as a pressure sensor:

A device for measuring pressure with a frequency output, containing an elastic sensitive element filled with gas and connected through a separator to the measured medium, and a frequency meter, characterized in that, in order to improve the accuracy of measurements, it uses a resonator cell of a gas quantum generator as an elastic sensitive element.

In conclusion, it should be noted that lasers are the main research tool in a new field of physics - nonlinear optics, which owes its very appearance to powerful lasers

Bohr's theory made it possible to explain the existence of line spectra.

The emission (or absorption) spectrum is a set of waves of certain frequencies that an atom of a given substance emits (or absorbs).

Spectra are solid, line, and striped.

Continuous spectra emit all substances that are in the solid or liquid state. The continuous spectrum contains waves of all frequencies of visible light and therefore looks like a colored band with a smooth transition from one color to another in this order: red, orange, yellow, green, blue and purple (every hunter wants to know where the pheasant is sitting).

Line spectra emit all substances in the atomic state. Atoms of all substances radiate sets of waves of quite definite frequencies peculiar only to them. As each person has his own personal fingerprints, so the atom of a given substance has its own, characteristic spectrum only for him. Line emission spectra look like colored lines separated by gaps. The nature of line spectra is explained by the fact that the atoms of a particular substance have only their own stationary states with their own characteristic energy, and, consequently, their own set of pairs of energy levels that an atom can change, i.e., an electron in an atom can only transfer from one specific orbits to other, well-defined orbits for a given chemical.

Striped spectra are emitted by molecules. Striped spectra look like line spectra, only instead of individual lines, separate series of lines are observed, perceived as separate bands. It is characteristic that whichever spectrum is emitted by these atoms is the same absorbed, i.e., the emission spectra coincide with the absorption spectra in terms of the set of emitted frequencies. Since atoms of different substances correspond to spectra peculiar only to them, there is a way to determine the chemical composition of a substance by studying its spectra. This method is called spectral analysis. Spectral analysis is used to determine the chemical composition of fossil ores during mining, to determine the chemical composition of stars, atmospheres, planets; is the main method for monitoring the composition of a substance in metallurgy and mechanical engineering.
№2 Laboratory work."Measurement of EMF and internal resistance of a current source using an ammeter and a voltmeter".

The purpose of the work: to measure the EMF and internal resistance of a current source using an ammeter and a voltmeter.

Necessary equipment: current source, ammeter, voltmeter, rheostat, key, connecting wires.

Ticket 24. Photoelectric effect and its laws. Einstein's equation for the photoelectric effect and Planck's constant. Application of the photoelectric effect in technology.

In 1900, the German physicist Max Planck hypothesized that light is emitted and absorbed in separate portions - quanta (or photons). The energy of each photon is determined by the formula E = hv, where h is Planck's constant equal to , v is the frequency of light. Planck's hypothesis explained many phenomena: in particular, the phenomenon of the photoelectric effect, discovered in 1887 by the German scientist Heinrich Hertz and experimentally studied by the Russian scientist A. G. Stoletov. The photoelectric effect is the phenomenon of the emission of electrons by a substance under the influence of light.
As a result of the research, three laws of the photoelectric effect were established.
1. The strength of the saturation current is directly proportional to the intensity of light radiation incident on the surface of the body.
2. The maximum kinetic energy of photoelectrons increases linearly with the frequency of light and depends on its intensity.
3. If the frequency of light is less than a certain minimum frequency defined for a given substance, then the photoelectric effect does not occur.
The dependence of photocurrent on voltage is shown in Figure 51.

The theory of the photoelectric effect was created by the German scientist A. Einstein in 1905. Einstein's theory is based on the concept of the work function of electrons from a metal and the concept of quantum light emission. According to Einstein's theory, the photoelectric effect has the following explanation: by absorbing a quantum of light, an electron acquires energy. When leaving the metal, the energy of each electron decreases by a certain amount, which is called the work function (Avy). The work function is the work required to remove an electron from a metal. Maximum energy

electrons after the escape (if there are no other losses) has the form: . This equation is called the Einstein equation.

Devices based on the principle of operation of which is the phenomenon of the photoelectric effect are called photocells. The simplest such device is a vacuum photocell. The disadvantages of such a photocell are: low current, low sensitivity to long-wave radiation, difficulty in manufacturing, impossibility of use in AC circuits. It is used in photometry for measuring luminous intensity, brightness, illumination, in cinema for sound reproduction, in phototelegraphs and phototelephones, in the management of production processes.
There are semiconductor photocells in which, under the influence of light, the concentration of current carriers changes. They are used in automatic control of electrical circuits (for example, in metro turnstiles), in alternating current circuits, as non-renewable current sources in watches, microcalculators, the first solar cars are being tested, they are used in solar batteries on artificial Earth satellites, interplanetary and orbital automatic stations .
The phenomenon of the photoelectric effect is associated with photochemical processes occurring under the action of light in photographic materials.
№2 A task to apply the law of conservation of momentum.

A diesel locomotive with a mass of 130 tons approaches a stationary train with a mass of 1170 tons at a speed of 2 m/s. At what speed will the train move after coupling with a diesel locomotive?

Rutherford's experiments on the scattering of alpha particles. Nuclear model of the atom.

It is known that the word "atom" in Greek means "indivisible". The English physicist J. Thomson developed (in the late 19th century) the first "model of the atom", according to which the atom is a positively charged sphere, inside which electrons floated. The model proposed by Thomson needed experimental verification, since the phenomena of radioactivity and the photoelectric effect could not be explained using Thomson's model of the atom. Therefore, in 1911, Ernest Rutherford conducted a series of experiments to study the composition and structure of atoms. In these experiments, a narrow beam a -particles emitted by a radioactive substance was directed to a thin gold foil. Behind it was placed a screen capable of glowing under the impact of fast particles. It was found that the majority a -particles deviates from rectilinear propagation after passing through the foil, i.e., it is scattered, and some a -particles are discarded by 180 0 .

Trajectories a- particles flying at different distances from the nucleus

lasers

Based on the quantum theory of radiation, quantum generators of radio waves and quantum generators of visible light - lasers - were built. Lasers produce coherent radiation of very high power. Laser radiation is very widely used in various fields of science and technology, for example, for communication in space, for recording and storing information (laser disks) and welding, and in medicine.

Emission and absorption of light by atoms

According to Bohr's postulates, an electron can be in several definite orbits. Each orbit of an electron corresponds to a certain energy. When an electron moves from a near to a far orbit, the atomic system absorbs a quantum of energy. When moving from a more distant orbit of an electron to a nearer orbit in relation to the nucleus, the atomic system emits a quantum of energy.

Spectra

Bohr's theory made it possible to explain the existence of line spectra.
Formula (1) gives a qualitative idea of ​​why the atomic emission and absorption spectra are line-like. In fact, an atom can only emit waves of those frequencies that correspond to the differences in energy values E 1 , E 2 , . . . , En ,. . That is why the radiation spectrum of atoms consists of separately located sharp bright lines. At the same time, an atom can absorb not any photon, but only the one with energy hν which is exactly equal to the difference E n − E k some two allowed energy values E n and E k. Moving into a state of higher energy E n, atoms absorb exactly the same photons that they are able to emit during the reverse transition to the initial state E k. Simply put, atoms take from the continuous spectrum those lines that they themselves emit; that is why the dark lines of the absorption spectrum of a cold atomic gas are exactly in those places where the bright lines of the emission spectrum of the same gas in a heated state are located.

continuous spectrum

Spectrum- the distribution of energy emitted or absorbed by a substance, according to frequencies or wavelengths.

If a prism is placed on the path of a beam of sunlight penetrating through a narrow long rectangular slit, then on the screen we will see not an image of the slit, but a stretched colored strip with a gradual transition of colors from red to violet - the spectrum. This phenomenon was observed by Newton. This means that the composition of sunlight includes electromagnetic waves of various frequencies. Such a spectrum is called solid.

If light is passed through a prism, which is emitted by a heated gas, then the spectrum will look like separate colored lines on a black background. Such a spectrum is called line emission spectrum. This means that the heated gas emits electromagnetic waves with a certain set of frequencies. Moreover, each chemical element emits a characteristic spectrum that is different from the spectra of other elements.

If light passes through a gas, then dark lines appear - line absorption spectrum.

Spectral analysis- a method for determining the qualitative and quantitative composition of a substance, based on obtaining and studying its spectra.

Regularities of radiation of atoms

Light emission occurs when an electron in an atom passes from the highest energy level E k to one of the lower energy levels E n (k > n). The atom in this case emits a photon with energy

The absorption of light is the reverse process. An atom absorbs a photon, passes from a lower state k to a higher state n (n > k). In this case, the atom absorbs a photon with energy