In nature, radiation has long been known that differs in nature from all known types of radiation (thermal radiation, reflection, light scattering, etc.). This radiation is luminescent radiation, examples of which are the glow of bodies when they are irradiated with visible, ultraviolet and x-rays, -radiation, etc. Substances capable of glowing under the action of various kinds of excitations are called phosphors.

Luminescence- non-equilibrium radiation, excess at a given temperature over the thermal radiation of the body and having a duration greater than the period of light oscillations. The first part of this definition leads to the conclusion that luminescence is not thermal radiation (see § 197), since any body at a temperature above 0 K radiates electromagnetic waves, and such radiation is thermal. The second part shows that luminescence is not such a type of glow as reflection and scattering of light, bremsstrahlung of charged particles, etc. The period of light oscillations is approximately 10 -15 s, so the duration by which the glow can be attributed to luminescence is longer - approximately 10 -10 s. sign

The glow duration makes it possible to distinguish luminescence from other nonequilibrium processes. Thus, on the basis of this feature, it was possible to establish that the Vavilov-Cherenkov radiation (see §189) cannot be attributed to luminescence.

Depending on the methods of excitation, there are: photoluminescence(under the influence of light), x-ray luminescence(under the influence of x-rays), cathodoluminescence(under the influence of electrons), electroluminescence(under the action of an electric field), radioluminescence(when excited by nuclear radiation, for example,  radiation, neutrons, protons), chemiluminescence(during chemical transformations), triboluminescence(when rubbing and splitting some crystals, such as sugar). According to the duration of the glow conditionally distinguish: fluorescence(t10 -8 s) and phosphorescence- a glow that continues for a noticeable period of time after the cessation of excitation.

The first quantitative study of luminescence was carried out over a hundred years ago. J. Stokes, who formulated the following rule in 1852: the wavelength of luminescent radiation is always greater than the wavelength of the light that excited it (Fig. 326). From a quantum point of view, Stokes' rule means that the energy hv incident photon is partially spent on some non-optical processes, i.e.

hv=hv lum +E,

whence v lum , which follows from the formulated rule.

The main energy characteristic of luminescence is energy outlet, introduced by S. I. Vavilov in 1924 - the ratio of the energy emitted by the phosphor during full illumination to the energy absorbed by it. Typical for organic luminophores (on the example of a fluorescein solution) dependence of the energy yield  on the wavelength  of the exciting light is shown in fig. 327. It follows from the figure that at first  grows in proportion to , and then, reaching a maximum value, it quickly drops to zero with a further increase To(Vavilov's law). The energy yield for different phosphors varies within fairly wide limits, its maximum value can reach about 80%.

Solids, which are effectively luminescent artificially prepared crystals with foreign impurities, are called crystal phosphors. Using the example of crystal phosphors, let us consider the mechanisms of the appearance of luminescence from the point of view zone theory solid bodies. Between the valence band and the conduction band of crystal phosphorus are the impurity levels of the activator (Fig. 328). At

When a photon with energy hv is absorbed by the activator atom, the electron from the impurity level is transferred to the conduction band, moves freely through the crystal until it meets the activator ion and recombines with it, passing again to the impurity level. Recombination is accompanied by the emission of a luminescent luminescence quantum. The glow time of the phosphor is determined by the lifetime of the excited state of the activator atoms, which usually does not exceed billionths of a second. Therefore, the glow is short-lived and disappears almost immediately after the cessation of irradiation.

For a long-term glow (phosphorescence) to occur, crystal phosphorus must also contain capture centers, or traps for electrons, which are unfilled local levels (for example, Jl 1 and L 2) lying near the bottom of the conduction band (Fig. 329). They can be formed by impurity atoms, atoms in interstices, etc. Under the action of light, the activator atoms are excited, i.e., electrons from the impurity level pass into the conduction band and become free. However, they are captured by traps, as a result of which they lose their mobility and, consequently, their ability to recombine with the activator ion. The release of an electron from a trap requires the expenditure of a certain energy, which electrons can receive, for example, from thermal vibrations of the lattice. The electron released from the trap enters the conduction band and moves through the crystal until it is either recaptured by the trap or recombines with the activator ion.

In the latter case, a quantum of luminescent radiation appears. The duration of this process is determined by the residence time of electrons in traps.

The phenomenon of luminescence has been widely used in practice, for example luminescent analysis - a method for determining the composition of a substance by its characteristic glow. This method, being very sensitive (approximately 10 -10 g / cm 3), makes it possible to detect the presence of negligible impurities and is used in the finest studies in biology, medicine, the food industry, etc. Fluorescent flaw detection allows you to detect the thinnest cracks on the surface of machine parts and other products (the surface under study is covered for this with a luminescent solution, which remains in the cracks after removal).

Phosphors are used in fluorescent lamps, are the active medium of optical quantum generators (see § 233) and scintillators (to be discussed below), are used in electron-optical converters (see § 169), are used to create emergency and camouflage lighting and for the manufacture of luminous indicators of various devices.

electromagnetic radiation. Application methods spectral analysis.

Radiation energy.

The light source must consume energy. Light is electromagnetic waves with a wavelength of 4 10-7 - 8 10-7 m. Electromagnetic waves emitted at fast motion charged particles. These charged particles are part of atoms. But, without knowing how the atom is arranged, nothing reliable can be said about the mechanism of radiation. It is only clear that there is no light inside an atom, just as there is no sound in a piano string. Like a string that begins to sound only after a hammer strike, atoms give birth to light only after they are excited.
In order for an atom to radiate, it needs to transfer energy. By radiating, an atom loses the energy it has received, and for the continuous glow of a substance, an influx of energy to its atoms from the outside is necessary.

thermal radiation. The simplest and most common type of radiation is thermal radiation, in which the loss of energy by atoms for the emission of light is compensated by energy thermal motion atoms or (molecules) radiating body.
AT early XIX in. it was found that above (in wavelength) the red part of the spectrum visible light the infrared part of the spectrum is invisible to the eye, and below the violet part of the visible light spectrum is the invisible ultraviolet part of the spectrum.
Wavelengths infrared radiation are enclosed within the range from 3 10-4 to 7.6 10-7 m. characteristic property this radiation is its thermal effect. The source of infrared rays is any body. The intensity of this radiation is the higher, the higher the temperature of the body. The higher the body temperature, the faster the atoms move. When fast atoms (molecules) collide with each other, some of them kinetic energy turns into excitation energy of atoms, which then emit light.

Infrared radiation is examined using thermocouples and bolometers. The principle of operation of night vision devices is based on the use of infrared radiation.
The heat source of radiation is the Sun, as well as an ordinary incandescent lamp. The lamp is a very convenient, but uneconomical source. Only about 12% of the total energy released in the lamp electric shock, is converted into light energy. The heat source of light is the flame. Grains of soot are heated by the energy released during the combustion of fuel, and emit light.

Electroluminescence. The energy needed by atoms to emit light can also be borrowed from non-thermal sources. When discharging in gases, the electric field imparts a large kinetic energy to the electrons. Fast electrons experience collisions with atoms. Part of the kinetic energy of electrons goes to the excitation of atoms. Excited atoms give off energy in the form of light waves. Due to this, the discharge in the gas is accompanied by a glow. This is electroluminescence.

cathodoluminescence. glow solids caused by their bombardment with electrons is called cathodoluminescence. The screens of cathode ray tubes glow due to cathodoluminescence.

Chemiluminescence. For some chemical reactions, going with the release of energy, part of this energy is directly spent on the emission of light. The light source remains cold (it has a temperature environment). This phenomenon is called chemiluminescence.

Photoluminescence. Light falling on a substance is partly reflected and partly absorbed. The energy of the absorbed light in most cases causes only heating of the bodies. However, some bodies themselves begin to glow directly under the action of the radiation incident on it. This is photoluminescence.

Light excites the atoms of matter (enlarges them internal energy), after which they are highlighted by themselves. For example, luminous paints, which cover many Christmas decorations, emit light after they are irradiated. Photoluminescence of solids, as well as special purpose- (generalized) phosphors, can be not only in the visible, but also in the ultraviolet and infrared ranges. The light emitted during photoluminescence has, as a rule, a longer wavelength than the light that excites the glow. This can be observed experimentally. If a light beam passed through a violet light filter is directed to a vessel with a fluorescent (organic dye), then this liquid begins to glow with green-yellow light, i.e., light of a longer wavelength than that of violet light.
The phenomenon of photoluminescence is widely used in fluorescent lamps. Soviet physicist S. I. Vavilov proposed to cover inner surface discharge tube with substances capable of glowing brightly under the action of short-wave radiation gas discharge.

Distribution of energy in the spectrum.

None of the sources gives monochromatic light, that is, light of a strictly defined wavelength. We are convinced of this by experiments on the decomposition of light into a spectrum with the help of a prism, as well as experiments on interference and diffraction.
The energy that light from the source carries with it is distributed in a certain way over the waves of all wavelengths that make up the light beam. We can also say that the energy is distributed over frequencies, since there is a simple relationship between wavelength and frequency: ђv = c.
The electromagnetic radiation flux density or intensity is determined by the energy attributable to all frequencies. To characterize the distribution of radiation over frequencies, you need to introduce a new value: the intensity per unit frequency interval. This value is called the spectral density of the radiation intensity.

You cannot rely on the eye when estimating the distribution of energy. The eye has a selective sensitivity to light: the maximum of its sensitivity lies in the yellow-green region of the spectrum. It is best to take advantage of the property of a black body to almost completely absorb light of all wavelengths. In this case, the energy of radiation (i.e., light) causes heating of the body. Therefore, it is sufficient to measure the body temperature and use it to judge the amount of energy absorbed per unit time.
An ordinary thermometer is too sensitive to be used successfully in such experiments. More sensitive temperature measuring instruments are needed. You can take an electric thermometer, in which sensing element made in the form of a thin metal plate. This plate must be covered with a thin layer of soot, which almost completely absorbs light of any wavelength.
The heat-sensitive plate of the instrument should be placed in one place or another in the spectrum. Everything visible spectrum length l from red rays to violet corresponds to the frequency range from IR to UV. The width corresponds to a small interval Av. By heating the black plate of the device, one can judge the density of the radiation flux per frequency interval Av. Moving the plate along the spectrum, we find that most of energy falls on the red part of the spectrum, and not on the yellow-green, as it seems to the eye.
Based on the results of these experiments, it is possible to plot the dependence of the spectral density of the radiation intensity on frequency. The spectral density of the radiation intensity is determined by the temperature of the plate, and the frequency is not difficult to find if the device used to decompose the light is calibrated, i.e., if it is known what frequency the given section of the spectrum corresponds to.
Plotting along the abscissa axis the values ​​of the frequencies corresponding to the midpoints of the Av intervals, and along the ordinate axis the spectral density of the radiation intensity, we obtain a series of points through which a smooth curve can be drawn. This curve gives a visual representation of the distribution of energy and the visible part of the spectrum of an electric arc.

Types of spectra.

Spectral composition of radiation various substances very varied. But, despite this, all spectra, as experience shows, can be divided into three types that differ from each other.

Continuous spectra.

The solar spectrum or the arc light spectrum is continuous. This means that all wavelengths are represented in the spectrum. There are no discontinuities in the spectrum, and a continuous multicolored band can be seen on the spectrograph screen.
Energy distribution over frequencies, i.e. Spectral density of radiation intensity, for various bodies different. For example, a body with a very black surface emits electromagnetic waves of all frequencies, but the spectral density of the radiation intensity versus frequency curve has a maximum at a certain frequency. The radiation energy attributable to very small and very high frequencies is negligible. As the temperature rises, the maximum spectral density of the radiation shifts towards short waves.
Continuous (or continuous) spectra, as experience shows, give bodies that are in solid or liquid state and highly compressed gases. To obtain a continuous spectrum, you need to heat the body to a high temperature.
The nature of the continuous spectrum and the very fact of its existence are determined not only by the properties of individual radiating atoms, but also in strong degree depends on the interaction of atoms with each other.
A continuous spectrum is also produced by high-temperature plasma. Electromagnetic waves are emitted by plasma mainly when electrons collide with ions.

Line spectra.

Let us introduce into the pale flame of a gas burner a piece of asbestos moistened with a solution of ordinary table salt. When observing a flame through a spectroscope, a bright yellow line flashes against the background of a barely distinguishable continuous spectrum of the flame. This yellow line is given by sodium vapor, which is formed during the splitting of sodium chloride molecules in a flame. On the spectroscope, one can also see a palisade of colored lines of varying brightness, separated by wide dark bands. Such spectra are called line spectra. Availability line spectrum means that the substance emits light only in well-defined wavelengths (more precisely, in certain very narrow spectral intervals). Each of the lines has a finite width.
Line spectra occur only in substances in the atomic state (but not molecular ones). In this case, light is emitted by atoms that practically do not interact with each other. This is the most fundamental, basic type of spectra. The main property of line spectra is that isolated atoms of a given chemical element emit strictly defined, non-repeating sequences of wavelengths. Two various elements there is no single sequence of wavelengths. Spectral bands appear at the output of a spectral device in place of the wavelength that is emitted from the source. Usually, to observe line spectra, the glow of vapors of a substance in a flame or the glow of a gas discharge in a tube filled with the gas under study is used.
With an increase in the density of an atomic gas, individual spectral lines expand and, finally, at very high density gas, when the interaction of atoms becomes significant, these lines overlap each other forming a continuous spectrum.

Striped spectra.

The striped spectrum consists of individual bands separated by dark gaps. With the help of a very good spectral apparatus, it can be found that each band is a collection a large number very closely spaced lines. Unlike line spectra, stripe spectra are created not by atoms, but by molecules that are not bound or weakly bound. bound friend with a friend.
To observe molecular spectra, as well as to observe line spectra, one usually uses the glow of vapors in a flame or the glow of a gas discharge.

Emission and absorption spectra.

All substances whose atoms are in excited state, radiate light waves, whose energy is distributed in a certain way over the wavelengths. The absorption of light by a substance also depends on the wavelength. So, red glass transmits waves corresponding to red light (l»8 10-5 cm), and absorbs all the rest.
If skip White light through a cold, non-radiating gas, dark lines appear against the background of the continuous spectrum of the source. The gas absorbs most intensely the light of precisely those wavelengths that it emits when it is very hot. The dark lines against the background of the continuous spectrum are the absorption lines, which together form the absorption spectrum.
There are continuous, line and striped emission spectra and the same number of absorption spectra.

Spectral analysis and its application.

It is important to know what the bodies around us are made of. Many methods have been devised to determine their composition. But the composition of stars and galaxies can only be known with the help of spectral analysis.

The method of determining the qualitative and quantitative composition of a substance by its spectrum is called spectral analysis. Spectral analysis is widely used in the search for minerals to determine chemical composition ore samples. In industry, spectral analysis makes it possible to control the compositions of alloys and impurities introduced into metals to obtain materials with desired properties. Line spectra play especially important role, because their structure is directly related to the structure of the atom. After all, these spectra are created by atoms that do not experience external influences. Therefore, getting acquainted with line spectra, we thereby take the first step towards studying the structure of atoms. By observing these spectra, scientists were able to "look" inside the atom. Here, optics comes into close contact with atomic physics.
The main property of line spectra is that the wavelengths (or frequencies) of the line spectrum of a substance depend only on the properties of the atoms of this substance, but are completely independent of the method of excitation of the luminescence of atoms. The atoms of any chemical element give off a spectrum unlike the spectra of all other elements: they are capable of emitting a strictly defined set of wavelengths.
Spectral analysis is based on this - a method for determining the chemical composition of a substance from its spectrum.

Like human fingerprints, line spectra have a unique personality. The uniqueness of the patterns on the skin of the finger often helps to find the criminal. In the same way, due to the individuality of the spectra, it is possible to determine the chemical composition of the body. Using spectral analysis, you can detect this element in the composition complex substance, even if its mass does not exceed 10-10. This is a very sensitive method.
The study of the line spectrum of a substance makes it possible to determine from which chemical elements it consists and in what quantity each element is contained in this substance.
The quantitative content of the element in the sample under study is determined by comparing the intensity of the individual lines of the spectrum of this element with the intensity of the lines of another chemical element, the quantitative content of which in the sample is known.
Quantitative analysis of the composition of a substance by its spectrum is difficult, since the brightness spectral lines depends not only on the mass of the substance, but also on the method of excitation of the glow. Yes, at low temperatures many spectral lines do not appear at all. However, under standard conditions for the excitation of luminescence, a quantitative spectral analysis can also be carried out.
The advantages of spectral analysis are high sensitivity and speed of results. With the help of spectral analysis, it is possible to detect the presence of gold in a sample weighing 6 10-7 g, while its mass is only 10-8 g. Determination of the steel grade by spectral analysis can be performed in several tens of seconds.
Spectral analysis allows you to determine the chemical composition celestial bodies billions of light years away from Earth. The chemical composition of the atmospheres of planets and stars, cold gas in interstellar space is determined by absorption spectra.
By studying the spectra, scientists were able to determine not only the chemical composition of celestial bodies, but also their temperature. The shift of spectral lines can be used to determine the speed of a celestial body.

At present, the spectra of all atoms have been determined and tables of spectra have been compiled. With the help of spectral analysis, many new elements were discovered: rubidium, cesium, etc. Elements were often given names according to the color of the most intense lines of the spectrum. Rubidium gives dark red, ruby ​​lines. The word cesium means "sky blue". This is the color of the main lines of the cesium spectrum.
It was with the help of spectral analysis that they learned the chemical composition of the Sun and stars. Other methods of analysis are generally impossible here. It turned out that the stars are composed of the same chemical elements that are found on Earth. It is curious that helium was originally discovered in the Sun and only then found in the Earth's atmosphere. The name of this element recalls the history of its discovery: the word helium means "sunny" in translation.
Due to its relative simplicity and versatility, spectral analysis is the main method for monitoring the composition of a substance in metallurgy, mechanical engineering, and the nuclear industry. With the help of spectral analysis, the chemical composition of ores and minerals is determined.
The composition of complex, mainly organic, mixtures is analyzed by their molecular spectra.
Spectral analysis can be performed not only from emission spectra, but also from absorption spectra. It is the absorption lines in the spectrum of the Sun and stars that make it possible to study the chemical composition of these celestial bodies. The brightly luminous surface of the Sun - the photosphere - gives a continuous spectrum. solar atmosphere selectively absorbs light from the photosphere, which leads to the appearance of absorption lines against the background of the continuous spectrum of the photosphere.
But the very atmosphere of the Sun emits light. During solar eclipses, when solar disk closed by the Moon, the lines of the spectrum are reversed. Instead of absorption lines in the solar spectrum, emission lines flash.
In astrophysics, spectral analysis is understood not only to determine the chemical composition of stars, gas clouds, etc., but also to find many other physical characteristics these objects: temperature, pressure, speed, magnetic induction.
In addition to astrophysics, spectral analysis is widely used in forensics, to investigate evidence found at a crime scene. Also, spectral analysis in forensics helps to determine the murder weapon and, in general, to reveal some particulars of the crime.
Spectral analysis is used even more widely in medicine. Here its application is very wide. It can be used for diagnosing, as well as in order to determine foreign substances in the human body.
Spectral analysis requires special spectral instruments, which we will consider further.

Spectral devices.

For an accurate study of spectra, such simple devices as a narrow slit limiting the light beam and a prism are no longer sufficient. Instruments are needed that give a clear spectrum, i.e., instruments that separate the waves well various lengths and non-overlapping individual sections spectrum. Such devices are called spectral devices. Most often, the main part of the spectral apparatus is a prism or diffraction grating.
Consider the scheme of the device of the prism spectral apparatus. The studied radiation first enters the part of the device called the collimator. The collimator is a tube, at one end of which there is a screen with a narrow slit, and at the other - a converging lens. The gap is on focal length from the lens. Therefore, a divergent light beam that enters the lens from the slit exits it in a parallel beam and falls on the prism.
Because different frequencies correspond to different refractive indices, then parallel beams emerge from the prism, not coinciding in direction. They fall on the lens. At the focal length of this lens is a screen - frosted glass or photographic plate. The lens focuses parallel beams of rays on the screen, and instead of a single image of the slit, whole line images. Each frequency (narrow spectral interval) has its own image. All these images together form a spectrum.
The described instrument is called a spectrograph. If instead of a second lens and a screen, a telescope is used for visual observation of spectra, then the device is called a spectroscope. Prisms and other details of spectral devices are not necessarily made of glass. Instead of glass, transparent materials such as quartz, rock salt, etc. are also used.

Thermal radiation and luminescence.

Energy consumed luminous body for radiation, can be replenished from various sources. Phosphorus oxidized in air glows due to the energy released during chemical transformation. This kind of light is called chemiluminescence. The glow that comes from various types independent gas discharge is called electroluminescence. The glow of solids caused by their bombardment with electrons is called cathodoluminescence. Emission of radiation by a body of a certain wavelength characteristic of it λ 1 can be caused by irradiating this body (or having previously irradiated it) with radiation of a wavelength λ 1 less than λ 2. Such processes are combined under the name of photoluminescence (Luminescence is called radiation, excess over the thermal radiation of the body at a given temperature and having a duration significantly exceeding the period of the emitted waves. Luminescent substances are called phosphors. ).

Figure 8. 1 Chemiluminescence

Figure 8. 2 Photoluminescence

Figure 8. 3 Electroluminescence.

The most common is the glow of bodies due to their heating. This type of glow is called thermal (or temperature) radiation. Thermal radiation occurs at any temperature, however, at low temperatures, practically only long (infrared) electromagnetic waves are emitted.

Let us surround the radiating body with an impenetrable shell with a perfectly reflecting surface (Fig.).

The radiation, falling on the body, will be absorbed by it (partially or completely). Consequently, there will be a continuous exchange of energy between the body and the radiation filling the shell. If the distribution of energy between the body and radiation remains unchanged for each wavelength, the state of the body-radiation system will be in equilibrium. Experience shows that the only type of radiation that can be in equilibrium with radiating bodies is thermal radiation. All other types of radiation are non-equilibrium.

The ability of thermal radiation to be in equilibrium with radiating bodies is due to the fact that its intensity increases with increasing temperature. Let us assume that the balance between the body and radiation (see Fig.) is violated and the body emits more energy than it absorbs.

Then the internal energy of the body will decrease, which will lead to a decrease in temperature. This, in turn, will cause a decrease in the amount of energy emitted by the body. The temperature of the body will decrease until the amount of energy radiated by the body becomes equal to the number absorbed energy. If the equilibrium is disturbed in the other direction, i.e., the amount of radiated energy is less than absorbed, the temperature of the body will increase until equilibrium is established again. Thus, an imbalance in the body-radiation system causes the occurrence of processes that restore the balance.

The situation is different in the case of any of the types of luminescence. Let's show this on the example of chemiluminescence. As long as the chemical reaction that causes the radiation proceeds, the radiating body moves further and further away from its original state. The absorption of radiation by the body will not change the direction of the reaction, but, on the contrary, will lead to a faster (due to heating) reaction in the original direction. Equilibrium will be established only when the entire supply of reacting substances and Luminescence is used up.

conditioned chemical processes, will be replaced by thermal radiation.

So, of all types of radiation, only thermal radiation can be in equilibrium. The laws of thermodynamics apply to equilibrium states and processes. Consequently, thermal radiation must also obey some general patterns arising from the principles of thermodynamics. It is to the consideration of these regularities that we turn.

8.2 Kirchhoff's law.

Let us introduce some characteristics of thermal radiation.

Energy flow (any frequency), emitted by a unit surface of a radiating body per unit time in all directions(within a solid angle 4π), called energy luminosity of the body (R) [R] = W/m2 .

Radiation consists of waves of different frequencies (ν). Let us denote the energy flux emitted by a unit surface of the body in the frequency range from ν to ν + dv, through d R v. Then at this temperature.

where - spectral density energy luminosity, or emissivity of the body .

Experience shows that the emissivity of a body depends on the temperature of the body (for each temperature, the maximum radiation lies in its own frequency range). Dimension .

Knowing the emissivity, we can calculate energy luminosity:

Let a stream of radiant energy dФ fall on an elementary area of ​​the body surface, due to electromagnetic waves, the frequencies of which are contained in the interval dν. Part of this flow will be absorbed by the body. Dimensionless

called absorption capacity of the body . It also strongly depends on temperature.

By definition, it cannot be greater than one. For a body that completely absorbs radiation of all frequencies, . Such a body is called absolutely black (this is an idealization).

The body for which and less than one for all frequencies,called gray body (this is also an idealization).

There is a certain relationship between the emitting and absorbing ability of the body. Let's mentally carry out the following experiment.

Let there be three bodies inside a closed shell. The bodies are in a vacuum, therefore, the exchange of energy can occur only due to radiation. Experience shows that after some time such a system will come to a state of thermal equilibrium (all bodies and the shell will have the same temperature).

In this state, the body, which has a greater radiative capacity, loses per unit time and more energy, but, therefore, this body must also have a greater absorbing capacity:

Gustav Kirchhoff in 1856 formulated law and suggested black body model .

The ratio of emissivity to absorptivity does not depend on the nature of the body, it is the same for all bodies.(universal)function of frequency and temperature.

where f( - generic function Kirchhoff.

This function has a universal, or absolute, character.

The quantities and , taken separately, can change extremely strongly when moving from one body to another, but their ratio constantly for all bodies (at a given frequency and temperature).

For an absolutely black body , =1 , therefore, for it f( , i.e. Kirchhoff's universal function is nothing but the radiance of a completely black body.

Absolutely black bodies do not exist in nature. Soot or platinum black have absorbing power , 1 , but only in a limited frequency range. However, a cavity with a small opening is very close in its properties to a completely black body. The beam that got inside, after multiple reflections, is necessarily absorbed, and the beam of any frequency.

The emissivity of such a device (cavity) is very close to f(ν ,T). Thus, if the walls of the cavity are maintained at a temperature T, then the radiation emitted from the hole is very close in spectral composition to blackbody radiation at the same temperature.

Expanding this radiation into a spectrum, one can find experimental view functions f(ν ,T)(Fig. 1.3), with different temperatures T 3 > T 2 > T 1 .

The area covered by the curve gives the energy luminosity of a black body at the appropriate temperature.

These curves are the same for all bodies.

The curves are similar to the velocity distribution function of molecules. But there, the areas covered by the curves are constant, while here, with increasing temperature, the area increases significantly. This suggests that energy compatibility is highly dependent on temperature. Maximum radiation (emissivity) with increasing temperature is shifting towards higher frequencies.

The energy expended by a luminous body for radiation can be replenished from various sources. Phosphorus oxidized in air glows due to the energy released during chemical transformation. This kind of light is called chemiluminescence.

The glow that occurs during various types of independent gas discharge is called electroluminescence. The glow of solids caused by their bombardment by electrons is called cathode-luminum and non-scene. The emission of radiation by a body of a certain wavelength λ 1 characteristic of it can be caused by irradiating this body (or having previously irradiated it) with radiation of a wavelength λ 2 that is less than λ 1 . Such processes are combined under the name of photoluminescence.

The most common is the glow of bodies due to their heating. This type of glow is called thermal (or temperature) radiation. Thermal radiation occurs at any temperature, however, at low temperatures, practically only long (infrared) electromagnetic waves are emitted.

Let us surround the radiating body with an impenetrable shell with a perfectly reflective surface (Fig. 154). Remove the air from the shell. The radiation reflected by the shell, falling on the body, will be absorbed by it (partially or completely). Consequently, there will be a continuous exchange of energy between the body and the radiation filling the shell. If the distribution of energy between the body and radiation remains unchanged for each wavelength, the state of the body-radiation system will be in equilibrium. Experience shows that the only type of radiation that can be in equilibrium with radiating bodies is thermal radiation. All other types of radiation are non-equilibrium.

The ability of thermal radiation to be in equilibrium with radiating bodies is due to the fact that its intensity increases with increasing temperature. Let us assume that the balance between the body and radiation (see Fig. 1) is disturbed and the body emits more energy than it absorbs. Then the internal energy of the body will decrease, which will lead to a decrease in temperature. This, in turn, will cause a decrease in the amount of energy emitted by the body. The temperature of the body will decrease until the amount of energy emitted by the body becomes equal to the amount of energy absorbed. If the equilibrium is disturbed in the other direction, i.e., the amount of radiated energy is less than absorbed, the temperature of the body will increase until equilibrium is established again. Thus, an imbalance in the body-radiation system causes the occurrence of processes that restore the balance.

The situation is different in the case of any of the types of luminescence. Let's show this on the example of chemiluminescence. As long as the chemical reaction that causes the radiation proceeds, the radiating body moves further and further away from its original state. The absorption of radiation by the body will not change the direction of the reaction, but, on the contrary, will lead to a faster (due to heating) reaction in the original direction. Equilibrium will be established only when the entire supply of reacting substances is used up and the luminescence due to chemical processes is replaced by thermal radiation.

So, of all types of radiation, only thermal radiation can be in equilibrium. The laws of thermodynamics apply to equilibrium states and processes. Consequently, thermal radiation must also obey some general laws arising from the principles of thermodynamics. It is to the consideration of these regularities that we turn.

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>> Types of radiation. Sources of light

§ 80 TYPES OF RADIATIONS. SOURCES OF LIGHT

Light is a stream of electromagnetic waves with a wavelength of 4 10 -7 -8 10 -7 m. Electromagnetic waves are emitted during the accelerated movement of charged particles. These charged particles are part of the atoms that make up matter. But, without knowing how the atom is arranged, nothing reliable can be said about the mechanism of radiation. It is only clear that there is no light inside an atom, just as there is no sound in a piano string. Like a string that begins to sound only after a hammer strike, atoms can only "give birth" to light after they are excited.

In order for an atom to begin to radiate, it needs to transfer a certain amount of energy. By radiating, the atom loses the energy it has received, and for the continuous glow of the substance, an influx of energy to its atoms from the outside is necessary.

Thermal radiation. The simplest and most common type of radiation is thermal radiation, in which the energy losses of atoms for the emission of light are compensated by the energy of thermal motion by the atom (or molecules) of the radiating body. Thermal radiation is the radiation of heated bodies. The higher the temperature of the topic, the faster the atoms move in it. When fast atoms (or molecules) collide with each other, part of their kinetic energy is used to excite atoms, which then emit light and pass into an unexcited state.

Thermal sources of radiation are, for example, the Sun and an ordinary incandescent lamp. The lamp is a very convenient, but inefficient source of light. Only about 12% of all the energy released in the lamp filament by electric current is converted into light energy. Finally, a flame is also a thermal source of light. Grains of soot (fuel particles that did not have time to burn) are heated by the energy released during the combustion of the fuel and emit light.

Electroluminescence. The energy needed by atoms to emit light can also come from non-thermal sources. When discharging in gases, the electric field imparts a large kinetic energy to the electrons. Fast electrons experience inelastic collisions with atoms. Part of the kinetic energy of electrons goes to the excitation of atoms. Excited atoms give off energy in the form of light waves. As a result, the discharge in the gas is accompanied by a glow. This is electroluminescence.

The northern lights are also a manifestation of electroluminescence. Streams of charged particles emitted by the Sun are captured magnetic field Earth. They excite magnetic poles The Earth's atoms are the upper layers of the atmosphere, which is why these layers glow. The phenomenon of electroluminescence is used in tubes for advertising inscriptions.

cathodoluminescence. The glow of solids caused by their bombardment with electrons is called cathodoluminescence. Thanks to cathodoluminescence, the screens of the cathode ray tubes of the TV glow.

Chemiluminescence. In some chemical reactions that go with the release of energy, part of this energy is directly spent on the emission of light. The light source remains cold (it has ambient temperature). This phenomenon is called chemiluminescence. Almost all of you are probably familiar with it. In the summer in the forest you can see an insect - a firefly at night. A small green "flashlight" "burns" on his body. You won't burn your fingers by catching a firefly. A luminous spot on its back has almost the same temperature as ambient air. Other living organisms also have the property of glowing: bacteria, insects, many fish that live at great depths. Quite often, pieces of rotting wood glow in the dark.

Photoluminescence. Light falling on a substance is partly reflected and partly absorbed. The energy of the absorbed light in most cases causes only heating of the bodies. However, some bodies themselves begin to glow directly under the action of the radiation incident on them. This is photoluminescence. Light excites the atoms of matter (increases their internal energy), and after that they are highlighted by themselves. For example, luminous paints used to cover Christmas decorations emit light after they are irradiated.

Vavilov Sergey Ivanovich (1891 -1951)- Soviet physicist, state and public figure, President of the Academy of Sciences of the USSR in 1945-1951. Main scientific works are devoted to physical optics, and primarily to photoluminescence. Under his leadership, a technology for the manufacture of fluorescent lamps was developed and a method for luminescent analysis of the chemical composition of substances was developed. Under his leadership, P. A. Cherenkov opened in 1934 light emission electrons moving through a medium at a speed greater than the speed of light in that medium.

The light emitted during photoluminescence has, as a rule, a longer wavelength than the light that excites the glow. This can be observed experimentally. If a light beam is directed to a vessel with fluorescein (organic dye), passed through an optical filter, then this liquid begins to glow with green-yellow light, i.e., light with a longer wavelength than that of optical light.

The phenomenon of photoluminescence is widely used in fluorescent lamps. The Soviet physicist S. I. Vavilov proposed covering the inner surface of the discharge tube with substances capable of glowing brightly under the action of short-wave radiation from a gas discharge.

Fluorescent lamps are about 3-4 times more economical than conventional incandescent lamps.

Of the listed main types of radiation, the most common is thermal radiation.

1. What light sources do you know!
2. What types of radiations affected you in the past day!

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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