The significance of luminescence of solids in nature. Electromagnetic radiation

Radiation has long been known in nature, which is different 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 irradiated with visible, ultraviolet and x-ray radiation, -radiation, etc. Substances capable of glowing under the influence of various types of excitations are called phosphors.

Luminescence- non-equilibrium radiation, which at a given temperature is excess over the thermal radiation of the body and has a duration longer 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 emits electromagnetic waves, and such radiation is thermal. The second part shows that luminescence is not a type of glow such as reflection and scattering of light, bremsstrahlung radiation of charged particles, etc. The period of light oscillations is approximately 10 -15 s, therefore the duration by which the glow can be classified as luminescence is longer - approximately 10 -10 s. Sign

The duration of the glow makes it possible to distinguish luminescence from other nonequilibrium processes. Thus, based on this criterion, 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 influence of an electric field), radioluminescence(when excited by nuclear radiation, for example -radiation, neutrons, protons), chemiluminescence(during chemical transformations), triboluminescence(when grinding and breaking certain crystals, such as sugar). Based on the duration of the glow, they are conventionally distinguished: 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 more than 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 of the incident photon is partially spent on some non-optical processes, i.e.

hv=hv lumen +E,

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

The main energy characteristic of luminescence is energy output, introduced by S.I. Vavilov in 1924 - the ratio of the energy emitted by a phosphor when fully illuminated to the energy absorbed by it. Typical for organic phosphors (using the example of a fluorescein solution), the dependence of the energy yield  on the wavelength  of the exciting light is shown in Fig. 327. From the figure it follows that at first  increases in proportion to , and then, reaching a maximum value, quickly decreases to zero with further increase TO(Vavilov’s law). The energy yield for various phosphors varies within fairly wide limits; its maximum value can reach approximately 80%.

Solids, which are effectively luminescent artificially prepared crystals with foreign impurities, are called crystal phosphors. Using crystal phosphors as an example, we will consider the mechanisms of luminescence occurrence from the point of view of the band theory of solids. Between the valence band and the conduction band of crystal phosphorus there are impurity levels of the activator (Fig. 328). At

When an activator atom absorbs a photon with energy hv, an electron from the impurity level is transferred to the conduction band and moves freely throughout the crystal until it encounters an activator ion and recombines with it, moving again to the impurity level. Recombination is accompanied by the emission of a luminescent 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 after the cessation of irradiation.

For 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 influence of light, the activator atoms are excited, i.e., electrons from the impurity level move 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 the electrons can obtain, for example, from thermal vibrations of the lattice. The electron released from the trap enters the conduction band and moves along the crystal until it is either captured again 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 the electrons in the traps.

The phenomenon of luminescence is widely used in practice, for example luminescence analysis - a method for determining the composition of a substance by its characteristic glow. This method, being very sensitive (approximately 10 -10 g/cm3), allows you to detect the presence of insignificant impurities and is used in the most delicate research in biology, medicine, the food industry, etc. Luminescent flaw detection allows you to detect the finest cracks on the surface of machine parts and other products (the surface being examined is covered with a luminescent solution, which, after removal, remains in the cracks).

Phosphors are used in fluorescent lamps, are the active medium of optical quantum generators (see § 233) and scintillators (will 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.

Introduction……………………………………………………………………………….2

Radiation mechanism………………………………………………………………………………..3

Energy distribution in the spectrum……………………………………………………….4

Types of spectra……………………………………………………………………………………….6

Types of spectral analyzes………………………………………………………7

Conclusion………………………………………………………………………………..9

Literature……………………………………………………………………………….11

Introduction

Spectrum is the decomposition of light into its component parts, rays of different colors.

Research method chemical composition various substances according to their line emission or absorption spectra are called spectral analysis. For spectral analysis a negligible amount of substance is required. Its speed and sensitivity have made this method indispensable both in laboratories and in astrophysics. Since each chemical element of the periodic table emits a line emission and absorption spectrum characteristic only for it, this makes it possible to study the chemical composition of the substance. The physicists Kirchhoff and Bunsen first tried to make it in 1859, building spectroscope. Light was passed into it through a narrow slit cut from one edge of the telescope (this pipe with a slit is called a collimator). From the collimator, the rays fell onto a prism covered with a box lined with black paper on the inside. The prism deflected the rays that came from the slit. The result was a spectrum. After this, they covered the window with a curtain and placed a lit burner at the collimator slit. Pieces of various substances were introduced alternately into the candle flame, and looked through the second telescope to the resulting spectrum. It turned out that the incandescent vapors of each element produced rays of a strictly defined color, and the prism deflected these rays to a strictly defined place, and therefore no color could mask the other. This led to the conclusion that a radically new method of chemical analysis had been found - using the spectrum of a substance. In 1861, based on this discovery, Kirchhoff proved the presence of a number of elements in the chromosphere of the Sun, laying the foundation for astrophysics.

Radiation mechanism

The light source must consume energy. Light is electromagnetic waves with a wavelength of 4*10 -7 - 8*10 -7 m. Electromagnetic waves are emitted at accelerated movement charged particles. These charged particles are part of atoms. But without knowing how the atom is structured, nothing reliable can be said about the radiation mechanism. 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 being struck by a hammer, atoms give birth to light only after they are excited.

In order for an atom to begin to radiate, energy must be transferred to it. When emitting, an atom loses the energy it receives, 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 energy lost by atoms to emit light is compensated by the energy of thermal motion of atoms or (molecules) of the emitting body. The higher the body temperature, the faster the atoms move. When fast atoms (molecules) collide with each other, part of them kinetic energy converted into excitation energy of atoms, which then emit light.

The thermal source of radiation is the Sun, as well as an ordinary incandescent lamp. The lamp is a very convenient, but low-cost source. Only about 12% of the total energy released in the lamp electric shock, is converted into light energy. The thermal source of light is a flame. Grains of soot heat up due to the energy released during fuel combustion and emit light.

Electroluminescence. The energy needed by atoms to emit light can also come from non-thermal sources. During a discharge in gases, the electric field imparts greater kinetic energy to the electrons. Fast electrons experience collisions with atoms. Part of the kinetic energy of electrons goes to excite atoms. Excited atoms release 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 bombardment by their electrons, is called cathodoluminescence. Thanks to cathodoluminescence, the screens of cathode ray tubes of televisions glow.

Chemiluminescence. For some chemical reactions, coming 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 chemioluminescence.

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

passed through a violet light filter, this liquid begins to glow with green-yellow light, i.e. light of a longer wavelength than violet light.

The phenomenon of photoluminescence is widely used in fluorescent lamps. Soviet physicist S.I. Vavilov proposed covering inner surface discharge tube with substances capable of glowing brightly under the influence of short-wave radiation gas discharge. Fluorescent lamps are approximately three to four times more economical than conventional incandescent lamps.

The main types of radiation and the sources that create them are listed. The most common sources of radiation are thermal.

Energy distribution in the spectrum

On the screen behind the refracting prism, monochromatic colors in the spectrum are arranged in the following order: red (which has the largest wavelength visible light wavelength (k = 7.6 (10-7 m and the smallest refractive index), orange, yellow, green, cyan, indigo and violet (having the shortest wavelength in the visible spectrum (f = 4 (10-7 m and the largest index) refraction). None of the sources produces 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 using a prism, as well as experiments on interference and diffraction.

The energy that light carries with it from the source is distributed in a certain way over the waves of all lengths that make up the light beam. We can also say that energy is distributed over frequencies, since there is a difference between wavelength and frequency. simple connection: v = c.

The flux density of electromagnetic radiation, or intensity /, is determined by the energy &W attributable to all frequencies. To characterize the frequency distribution of radiation, it is necessary to introduce a new quantity: the intensity per unit frequency interval. This quantity is called the spectral density of radiation intensity.

Spectral Density radiation flux can be found experimentally. To do this, you need to use a prism to obtain emission spectrum, For example, electric arc, and measure the radiation flux density falling on small spectral intervals of width Av.

An ordinary thermometer is too sensitive to be successfully used in such experiments. More sensitive instruments are needed to measure temperature. You can take an electric thermometer in which sensing element made in the form of a thin metal plate. This plate must be coated with a thin layer of soot, which almost completely absorbs light of any wavelength.

The heat-sensitive plate of the device should be placed in one or another place in the spectrum. Everything visible spectrum length l from red to violet rays corresponds to the frequency interval from v cr to y f. The width corresponds to a small interval Av. By heating the black plate of the device, one can judge the radiation flux density 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 construct a dependence curve spectral density radiation intensity versus frequency. The spectral density of 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, that is, if it is known what frequency a given part of the spectrum corresponds to.

By plotting along the abscissa axis the values of the frequencies corresponding to the midpoints of the intervals Av, and along the ordinate axis the spectral density of the radiation intensity, we obtain a number of points through which we can draw a smooth curve. This curve gives visual representation on the distribution of energy and the visible part of the spectrum of the electric arc.

Spectral devices. For precise research Spectra such simple devices as a narrow slit limiting the light beam and a prism are no longer sufficient. Instruments are needed that provide a clear spectrum, i.e., instruments that can well separate waves of different lengths and do not allow individual parts of the spectrum to overlap. Such devices are called spectral devices. Most often, the main part of the spectral apparatus is a prism or diffraction grating.

Let us consider the design diagram of a prism spectral apparatus. The radiation under study first enters a part of the device called a collimator. The collimator is a tube, at one end of which there is a screen with a narrow slit, and at the other - a collecting lens. The slit is at the focal length of the lens. Therefore, a diverging light beam incident on the lens from the slit emerges from it as a parallel beam and falls on the prism.

Since different frequencies correspond to different refractive indices, parallel beams that do not coincide in direction emerge from the prism. They fall on the lens. At the focal length of this lens there 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, the result is whole line images. Each frequency (narrow spectral interval) has its own image. All these images together form a spectrum.

The described device is called a spectrograph. If, instead of a second lens and screen, a telescope is used to visually observe spectra, then the device is called a spectroscope, described above. Prisms and other parts of spectral devices are not necessarily made of glass. Instead of glass, transparent materials such as quartz, rock salt, etc. are also used.

Types of spectra

The spectral composition of radiation from substances is very diverse. But, despite this, all spectra, as experience shows, can be divided into several types:

Continuous spectra. The solar spectrum or arc light spectrum is continuous. This means that the spectrum contains waves of all wavelengths. There are no breaks in the spectrum, and a continuous multi-colored strip can be seen on the spectrograph screen.

Energy distribution over frequencies, i.e. spectral density of radiation intensity, for different bodies various. For example, a body with a very black surface emits electromagnetic waves of all frequencies, but the curve of the dependence of the spectral density of radiation intensity on frequency has a maximum at a certain frequency. The radiation energy at very low and very high frequencies is negligible. With increasing temperature, the maximum spectral density of radiation shifts towards shorter waves.

Continuous (or continuous) spectra, as experience shows, are given by bodies located in solid or liquid state, as well as highly compressed gases. To obtain a continuous spectrum, the body must be heated 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 emitting atoms, but also in strong degree depend 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's add a piece of asbestos moistened with a solution of ordinary water into the pale flame of a gas burner. table salt.

When observing a flame through a spectroscope, a bright yellow line will flash against the background of the barely visible continuous spectrum of the flame. This yellow line is produced by sodium vapor, which is formed when the molecules of table salt are broken down in a flame. Each of them is a palisade of colored lines of varying brightness, separated by wide dark

stripes. Such spectra are called line spectra. Availability line spectrum means that the substance emits light only at certain wavelengths (more precisely, in certain very narrow spectral intervals). Each line has a finite width.

Line spectra give all substances in the gaseous atomic (but not molecular) state. 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.

Isolated atoms emit strictly defined wavelengths. Typically, to observe line spectra, the glow of vapor of a substance in a flame or the glow of a gas discharge in a tube filled with the gas under study is used.

As the density of the atomic gas increases, the individual spectral lines expand, and finally, with very high compression of the gas, when the interaction of atoms becomes significant, these lines overlap each other, forming a continuous spectrum.

Striped spectra. The banded spectrum consists of individual bands separated by dark spaces. With the help of a very good spectral apparatus it is possible

discover that each band represents a collection large number very closely spaced lines. Unlike line spectra, striped spectra are created not by atoms, but by molecules that are not bound or weakly bound to each other.

To observe molecular spectra, as well as to observe line spectra, the glow of vapor in a flame or the glow of a gas discharge is usually used.

Absorption spectra. All substances whose atoms are in an excited state emit light waves, the energy of which is distributed in a certain way over wavelengths. The absorption of light by a substance also depends on the wavelength. Thus, red glass transmits waves corresponding to red light and absorbs all others.

If you pass white light through a cold, non-emitting 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 highly heated. Dark lines against the background of a continuous spectrum are absorption lines that together form an absorption spectrum.

There are continuous, line and striped emission spectra and the same number of types of absorption spectra.

Line spectra play a special role 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, by becoming familiar with line spectra, we thereby take the first step towards studying the structure of atoms. By observing these spectra, scientists obtained

the opportunity to “look” inside the atom. Here optics comes into close contact with atomic physics.

Types of spectral analyzes

The main property of line spectra is that the wavelengths (or frequencies) of the line spectrum of any 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 the atoms. Atoms

any chemical element gives a spectrum that is not similar to the spectra of all other elements: they are capable of emitting a strictly defined set of wavelengths.

This is the basis of spectral analysis - a method of 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, there is

the ability to determine the chemical composition of the body. Using spectral analysis, you can detect this element as part of complex substance. This is a very sensitive method.

Currently known the following types spectral analyzes - atomic spectral analysis (ASA)(determines the elemental composition of a sample from atomic (ion) emission and absorption spectra), emission ASA(based on the emission spectra of atoms, ions and molecules excited by various sources of electromagnetic radiation in the range from g-radiation to microwave), atomic absorption SA(carried out using the absorption spectra of electromagnetic radiation by the analyzed objects (atoms, molecules, ions of matter in various states of aggregation)), atomic fluorescence SA, molecular spectral analysis (MSA) (molecular composition substances by molecular spectra of absorption, luminescence and Raman scattering of light.), quality ISA(it is enough to establish the presence or absence of analytical lines of the elements being determined. Based on the brightness of the lines during visual inspection, one can give a rough estimate of the content of certain elements in the sample), quantitative ISA(carried out by comparing the intensities of two spectral lines in the spectrum of the sample, one of which belongs to the element being determined, and the other (comparison line) to the main element of the sample, the concentration of which is known, or an element specially introduced at a known concentration).

MSA is based on a qualitative and quantitative comparison of the measured spectrum of the sample under study with the spectra of individual substances. Accordingly, a distinction is made between qualitative and quantitative ISA. MSA uses various types of molecular spectra, rotational [spectra in the microwave and long-wave infrared (IR) regions], vibrational and vibrational-rotational [absorption and emission spectra in the mid-IR region, Raman spectra, IR fluorescence spectra ], electronic, electronic-vibrational and electronic-vibrational-rotational [absorption and transmission spectra in the visible and ultraviolet (UV) regions, fluorescence spectra]. MSA allows analysis of small quantities (in some cases a fraction mcg and less) substances in different states of aggregation.

Quantitative analysis of the composition of a substance based on its spectrum is difficult, since the brightness of the spectral lines depends not only on the mass of the substance, but also on the method of excitation of the glow. Thus, at low temperatures, many spectral lines do not appear at all. However, subject to standard conditions for excitation of the glow, quantitative spectral analysis can also be carried out.

The most accurate of these tests is atomic absorption SA. The AAA technique is much simpler compared to other methods; it is characterized by high accuracy in determining not only small, but also large concentrations of elements in samples. AAA successfully replaces labor-intensive and time-consuming chemical methods analysis, not inferior to them in accuracy.

Conclusion

It was with the help of spectral analysis that the chemical composition of the Sun and stars was learned. Other methods of analysis are generally impossible here. It turned out that stars consist 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 “solar” in translation.

Due to its comparative simplicity and versatility, spectral analysis is the main method for monitoring the composition of a substance in metallurgy, mechanical engineering, and the nuclear industry. Using 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 - produces 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 atmosphere of the Sun itself emits light. During solar eclipses, when solar disk is blocked by the Moon, the spectrum lines are reversed. In place of absorption lines in the solar spectrum, emission lines flash.

In astrophysics, spectral analysis means not only the determination of the chemical composition of stars, gas clouds, etc., but also the determination of many

other physical characteristics of these objects: temperature, pressure, speed of movement, magnetic induction.

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

Express ASA methods are widely used in industry, agriculture, geology and many other areas of the national economy and science. ASA plays a significant role in nuclear technology, the production of pure semiconductor materials, superconductors, etc. More than 3/4 of all analyzes in metallurgy are performed using ASA methods. Using quantum meters, an operational procedure is carried out (within 2-3 min) control during melting in open-hearth and converter production. In geology and geological exploration To evaluate deposits, about 8 million analyzes are performed per year. ASA is used in environmental protection and soil analysis, forensics and medicine, seabed geology and compositional research. upper layers atmosphere, at

separation of isotopes and determination of the age and composition of geological and archaeological objects, etc.

So, spectral analysis is used in almost all the most important areas of human activity. Thus, spectral analysis is one of the most important aspects of the development of not only scientific progress, but also the very standard of human life.

Literature

Zaidel A.N., Fundamentals of spectral analysis, M., 1965,

Methods of spectral analysis, M, 1962;

Chulanovsky V.M., Introduction to molecular spectral analysis, M. - L., 1951;

Rusanov A.K., Fundamentals of quantitative spectral analysis of ores and minerals. M., 1971

Thermal radiation and luminescence.

The energy spent by a luminous body on radiation can be replenished from various sources. Phosphorus that oxidizes in air glows due to the energy released during the chemical transformation. This type of glow is called chemiluminescence. The glow that occurs when various types independent gas discharge is called electroluminescence. The glow of solids caused by the bombardment of electrons is called cathodoluminescence. Emission by a body of radiation of a certain wavelength characteristic of it λ 1 can be caused by irradiating this body (or having previously irradiated it) with radiation of the wavelength λ 1 less than λ 2. Such processes are combined under the name photoluminescence (Luminescence is radiation that is in excess of the thermal radiation of a body at a given temperature and has a duration that significantly exceeds 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, but at low temperatures almost only long (infrared) electromagnetic waves are emitted.

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

Radiation falling on a body is 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 energy distribution between the body and the 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 turn out to be nonequilibrium.

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 figure) is broken 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 body temperature will decrease until the amount of energy emitted by the body becomes equal to the number absorbed energy. If the equilibrium is disturbed in the other direction, i.e., the amount of energy emitted is less than that absorbed, the body temperature will increase until equilibrium is established again. Thus, an imbalance in the body-radiation system causes the emergence of processes that restore balance.

The situation is different in the case of any type of luminescence. Let us demonstrate this using the example of chemiluminescence. While the chemical reaction causing radiation is taking place, the radiating body moves further and further away from its original state. Absorption of radiation by a 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 the Glow is consumed.

conditional 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 obey certain general patterns, arising from the principles of thermodynamics. We will now move on to consider these patterns.

8.2 Kirchhoff's law.

Let us introduce some characteristics of thermal radiation.

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

Radiation consists of waves of different frequencies (ν). Let us denote the flow of energy emitted by a unit surface of a body in the frequency range from ν to ν + dν, through d Rν. Then at a given temperature.

Where - spectral density energetic luminosity, or body emissivity .

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 energetic luminosity:

Let a flux of radiant energy dФ fall on an elementary area of the body surface, caused by 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 depends greatly on temperature.

By definition it cannot be more 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 unity for all frequencies,called gray body (this is also an idealization).

There is a certain connection between the emissive and absorptive capacity of a body. Let's mentally conduct the following experiment.

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

In this state, a body with greater emissivity loses more energy per unit time, but, therefore, this body must also have greater absorption capacity:

Gustav Kirchhoff formulated in 1856 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( – universal 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. the universal Kirchhoff function is nothing more than the emissivity of a completely black body.

Absolutely black bodies do not exist in nature. Soot or platinum black has an absorptivity of 1, but only in a limited frequency range. However, a cavity with a small hole is very close in its properties to a completely black body. A beam that gets inside is necessarily absorbed after multiple reflections, and a beam of any frequency.

The emissivity of such a device (cavity) is very close to f(ν ,T). Thus, if the cavity walls are maintained at a temperature T, then radiation comes out of the hole, very close in spectral composition to the radiation of an absolutely black body at the same temperature.

By decomposing this radiation into a spectrum, one can find the experimental form of the function f(ν ,T)(Fig. 1.3), with different temperatures T 3 > T 2 > T 1 .

The area covered by the curve gives the energetic luminosity of a black body at the corresponding temperature.

These curves are the same for all bodies.

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

>> Types of radiation. Sources of light

§ 80 TYPES OF RADIATION. 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 by 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 structured, nothing reliable can be said about the radiation mechanism. 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 being struck by a hammer, atoms can “give birth” to light only after they are excited.

In order for an atom to begin to radiate, it needs to transfer a certain amount of energy. When emitting, an atom loses the energy it receives, 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 widespread type of radiation is thermal radiation, in which the energy lost by atoms to emit light is compensated by energy thermal movement atom (or molecules) of the radiating body. Thermal radiation is radiation from 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 goes to excite the atoms, which then emit light and go into a non-excited state.

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

Electroluminescence. The energy required for atoms to emit light can also come from non-thermal sources. During a discharge in gases, the electric field imparts greater kinetic energy to the electrons. Fast electrons experience inelastic collisions with atoms. Part of the kinetic energy of electrons goes to excite atoms. Excited atoms release 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 you magnetic poles Earth's atoms are in 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 the bombardment of electrons is called cathodoluminescence. Thanks to cathodoluminescence, television cathode ray tube screens glow.

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

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

Vavilov Sergei Ivanovich (1891 -1951)- Soviet physicist, state and public figure, President of the USSR Academy of Sciences in 1945-1951. Basic scientific works dedicated physical optics, and primarily photoluminescence. Under his leadership, the technology for manufacturing fluorescent lamps was developed and the method of luminescent analysis of the chemical composition of substances was developed. Under his leadership, P. A. Cherenkov opened in 1934. light emission electrons moving in a medium at a speed exceeding the speed of light in this medium.

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

The phenomenon of photoluminescence is widely used in fluorescent lamps. 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 approximately 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 radiation affected you in the past 24 hours!

Myakishev G. Ya., Physics. 11th grade: educational. for general education institutions: basic and profile. levels / G. Ya. Myakishev, B. V. Bukhovtsev, V. M. Charugin; edited by V. I. Nikolaeva, N. A. Parfentieva. - 17th ed., revised. and additional - M.: Education, 2008. - 399 p.: ill.

Calendar-thematic planning in physics, tasks and answers for schoolchildren online, courses for teachers in physics download

Lesson content lesson notes supporting frame lesson presentation acceleration methods interactive technologies Practice tasks and exercises self-test workshops, trainings, cases, quests homework controversial issues rhetorical questions from students Illustrations audio, video clips and multimedia photographs, pictures, graphics, tables, diagrams, humor, anecdotes, jokes, comics, parables, sayings, crosswords, quotes Add-ons abstracts articles tricks for the curious cribs textbooks basic and additional dictionary of terms other Improving textbooks and lessonscorrecting errors in the textbook updating a fragment in a textbook, elements of innovation in the lesson, replacing outdated knowledge with new ones Only for teachers perfect lessons calendar plan for a year guidelines discussion programs Integrated Lessons

Electromagnetic radiation. Application of spectral analysis methods.

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 are emitted by the accelerated movement of charged particles. These charged particles are part of atoms. But without knowing how the atom is structured, nothing reliable can be said about the radiation mechanism. 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 being struck by a hammer, atoms give birth to light only after they are excited.
In order for an atom to begin to radiate, energy must be transferred to it. When emitting, an atom loses the energy it receives, 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 energy lost by atoms to emit light is compensated by the energy of thermal motion of atoms or (molecules) of the emitting body.
IN early XIX V. It was discovered that above (in wavelength) the red part of the spectrum of visible light there is an infrared part of the spectrum invisible to the eye, and below the violet part of the spectrum of visible light there is an invisible ultraviolet part of the spectrum.
Wavelengths infrared radiation are contained within the range from 3·10-4 to 7.6·10-7 m. The most characteristic property this radiation is its thermal effect. The source of IR rays is any body. The higher the body temperature, the higher the intensity of this radiation. The higher the body temperature, the faster the atoms move. When fast atoms (molecules) collide with each other, part of their kinetic energy is converted into excitation energy of the atoms, which then emit light.

Infrared radiation is studied using thermocouples and bolometers. The operating principle of night vision devices is based on the use of infrared radiation.
The thermal source of radiation is the Sun, as well as an ordinary incandescent lamp. The lamp is a very convenient, but low-cost source. Only about 12% of the total energy released by electric current in a lamp is converted into light energy. The thermal source of light is a flame. Grains of soot heat up due to the energy released during fuel combustion and emit light.

Electroluminescence. The energy needed by atoms to emit light can also come from non-thermal sources. During a discharge in gases, the electric field imparts greater kinetic energy to the electrons. Fast electrons experience collisions with atoms. Part of the kinetic energy of electrons goes to excite atoms. Excited atoms release energy in the form of light waves. Due to this, the discharge in the gas is accompanied by a glow. This is electroluminescence.

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

Chemiluminescence. In some chemical reactions that release energy, part of this energy is directly spent on the emission of light. The light source remains cool (it is at ambient temperature). This phenomenon is called chemiluminescence.

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

Light excites the atoms of a substance (increases their internal energy), after which they are illuminated themselves. For example, the luminous paints that cover many Christmas tree decorations emit light after being 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, as a rule, has a longer wavelength than the light that excites the glow. This can be observed experimentally. If you direct a light beam passed through a violet filter onto 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 violet light.
The phenomenon of photoluminescence is widely used in fluorescent lamps. 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.

Energy distribution in the spectrum.

None of the sources produces 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 using a prism, as well as experiments on interference and diffraction.
The energy that light carries with it from the source is distributed in a certain way over the waves of all lengths that make up the light beam. We can also say that energy is distributed over frequencies, since there is a simple relationship between wavelength and frequency: ђv = c.
The flux density of electromagnetic radiation or intensity is determined by the energy at all frequencies. To characterize the frequency distribution of radiation, it is necessary to introduce a new quantity: the intensity per unit frequency interval. This quantity is called the spectral density of radiation intensity.

You cannot rely on your eye to estimate energy distribution. The eye has selective sensitivity to light: its maximum 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, radiation energy (i.e. light) causes heating of the body. Therefore, it is enough 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 successfully used in such experiments. More sensitive instruments are needed to measure temperature. You can take an electric thermometer, in which the sensitive element is made in the form of a thin metal plate. This plate must be coated with a thin layer of soot, which almost completely absorbs light of any wavelength.
The heat-sensitive plate of the device should be placed in one or another place in the spectrum. The entire visible spectrum of length l from red to violet rays 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 radiation flux density per frequency interval Av. Moving the plate along the spectrum, we will find that most of the energy is in the red part of the spectrum, and not in the yellow-green, as it seems to the eye.
Based on the results of these experiments, it is possible to construct a curve of the dependence of the spectral density of radiation intensity on frequency. The spectral density of 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, that is, if it is known what frequency a given part of the spectrum corresponds to.
By plotting along the abscissa axis the values of the frequencies corresponding to the midpoints of the intervals Av, and along the ordinate axis the spectral density of the radiation intensity, we obtain a number of points through which we can draw a smooth curve. This curve gives a visual representation of the distribution of energy and the visible part of the spectrum of the electric arc.

Types of spectra.

The spectral composition of radiation from various substances is very diverse. 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 arc light spectrum is continuous. This means that the spectrum contains waves of all wavelengths. There are no breaks in the spectrum, and a continuous multi-colored strip can be seen on the spectrograph screen.
The distribution of energy over frequencies, i.e., the spectral density of radiation intensity, is different for different bodies. For example, a body with a very black surface emits electromagnetic waves of all frequencies, but the curve of the spectral density of radiation intensity versus frequency has a maximum at a certain frequency. The radiation energy at very low and very high frequencies is negligible. With increasing temperature, the maximum spectral density of radiation shifts towards shorter waves.
Continuous (or continuous) spectra, as experience shows, are given by bodies in the solid or liquid state, as well as highly compressed gases. To obtain a continuous spectrum, the body must be heated 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 emitting atoms, but also to a strong extent depend 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's add a piece of asbestos moistened with a solution of ordinary table salt into the pale flame of a gas burner. When observing a flame through a spectroscope, a bright yellow line will flash against the background of the barely visible continuous spectrum of the flame. This yellow line is produced by sodium vapor, which is formed when the molecules of table salt are broken down in a flame. On the spectroscope you can also see a palisade of colored lines of varying brightness, separated by wide dark stripes. Such spectra are called line spectra. The presence of a line spectrum means that a substance emits light only at certain wavelengths (more precisely, in certain very narrow spectral intervals). Each line has a finite width.
Line spectra occur only for 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 same sequence of wavelengths. Spectral bands appear at the output of a spectral device at the location of the wavelength emitted from the source. Typically, to observe line spectra, the glow of vapor of a substance in a flame or the glow of a gas discharge in a tube filled with the gas under study is used.
As the density of the atomic gas increases, 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 banded spectrum consists of individual bands separated by dark spaces. With the help of a very good spectral apparatus one can discover that each band is a collection of a large number of very closely spaced lines. Unlike line spectra, striped spectra are created not by atoms, but by molecules that are not bound or weakly bound to each other.
To observe molecular spectra, as well as to observe line spectra, the glow of vapor in a flame or the glow of a gas discharge is usually used.

Emission and absorption spectra.

All substances whose atoms are in an excited state emit light waves, the energy of which is distributed in a certain way over wavelengths. The absorption of light by a substance also depends on the wavelength. Thus, red glass transmits waves corresponding to red light (l»8·10-5 cm), and absorbs all others.
If you pass white light through a cold, non-emitting 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 highly heated. Dark lines against the background of a continuous spectrum are absorption lines that together form an absorption spectrum.
There are continuous, line and striped emission spectra and the same number of types of absorption spectra.

Spectral analysis and its application.

Method for determining quality and quantitative composition The analysis of a substance by its spectrum is called spectral analysis. Spectral analysis is widely used in mineral exploration to determine the chemical composition of ore samples. In industry, spectral analysis makes it possible to control the composition of alloys and impurities introduced into metals to obtain materials with specified properties. Line spectra play a particularly 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, by becoming familiar 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 any 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 the atoms. The atoms of any chemical element give a spectrum that is not similar to the spectra of all other elements: they are capable of emitting strictly specific set wavelengths.
This is the basis of spectral analysis - a method of 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, thanks to the individuality of the spectra, it is possible to determine the chemical composition of the body. Using spectral analysis, it is possible to detect this element in the composition of a complex substance, even if its mass does not exceed 10-10. This is a very sensitive method.
Studying the line spectrum of a substance allows us to determine which chemical elements it consists and in what quantity each element is contained in a given substance.
The quantitative content of the element in the sample under study is determined by comparing the intensity separate lines spectrum of this element with the intensity of lines of another chemical element, the quantitative content of which in the sample is known.
Quantitative analysis of the composition of a substance based on its spectrum is difficult, since the brightness of the spectral lines depends not only on the mass of the substance, but also on the method of excitation of the glow. Yes, when low temperatures many spectral lines do not appear at all. However, subject to standard conditions for excitation of the glow, quantitative spectral analysis can also be carried out.
The advantages of spectral analysis are high sensitivity and speed of obtaining results. Using spectral analysis, it is possible to detect the presence of gold in a sample weighing 6·10-7 g, with its mass only 10-8 g. Determination of the steel grade by spectral analysis can be performed in a few tens of seconds.
Spectral analysis makes it possible to determine the chemical composition of celestial bodies located at distances of billions of light years from the Earth. Chemical composition of the atmospheres of planets and stars, cold gas in interstellar space determined from absorption spectra.
By studying the spectra, scientists were able to determine not only the chemical composition of celestial bodies, but also their temperature. By the displacement of spectral lines, one can determine the speed of movement of a celestial body.

Currently, the spectra of all atoms have been determined and tables of the spectra have been compiled. With the help of spectral analysis, many new elements were discovered: rubidium, cesium, etc. Elements were often given names in accordance with the color of the most intense lines in the spectrum. Rubidium produces dark red, ruby lines. The word cesium means "sky blue". This is the color of the main lines of the spectrum of cesium.
It was with the help of spectral analysis that the chemical composition of the Sun and stars was learned. Other methods of analysis are generally impossible here. It turned out that stars consist 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 “solar”.
Due to its comparative simplicity and versatility, spectral analysis is the main method for monitoring the composition of a substance in metallurgy, mechanical engineering, and the nuclear industry. Using 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. Bright glowing surface The sun's photosphere provides a continuous spectrum. The 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 atmosphere of the Sun itself emits light. During solar eclipses, when the solar disk is covered by the Moon, the lines of the spectrum are reversed. In place of the absorption lines in solar spectrum emission lines flash.
In astrophysics, spectral analysis means not only the determination of the chemical composition of stars, gas clouds, etc., but also the determination of many other things from the spectra physical characteristics these objects: temperature, pressure, speed, magnetic induction.
In addition to astrophysics, spectral analysis is widely used in forensic science to investigate evidence found at a crime scene. Also, spectral analysis in forensic science is very helpful in identifying the murder weapon and generally revealing some of the details of the crime.
Spectral analysis is used even more widely in medicine. Here its application is very great. It can be used for diagnosis, as well as to identify foreign substances in the human body.
Spectral analysis requires special spectral instruments, which we will consider further.

Spectral devices.

For 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 provide a clear spectrum, i.e., instruments that can well separate waves of different lengths and do not allow individual parts of the spectrum to overlap. Such devices are called spectral devices. Most often, the main part of the spectral apparatus is a prism or diffraction grating.
Let us consider the design diagram of a prism spectral apparatus. The radiation under study first enters a part of the device called a collimator. The collimator is a tube, at one end of which there is a screen with narrow gap, and on the other there is a converging lens. The slit is at the focal length of the lens. Therefore, a diverging light beam incident on the lens from the slit emerges from it as a parallel beam and falls on the prism.
Because different frequencies correspond various indicators refraction, then parallel beams that do not coincide in direction emerge from the prism. They fall on the lens. At the focal length of this lens there is a screen - frosted glass or photographic plate. The lens focuses parallel beams of rays on the screen, and instead of one image of the slit, a whole series of images is obtained. Each frequency (narrow spectral interval) has its own image. All these images together form a spectrum.
The described device is called a spectrograph. If, instead of a second lens and screen, a telescope is used to visually observe spectra, then the device is called a spectroscope. Prisms and other parts of spectral devices are not necessarily made of glass. Instead of glass, transparent materials such as quartz are also used. rock salt and etc.