The energy that a body loses due to thermal radiation is characterized by the following values.
Flux (F) - energy emitted per unit of time from the entire surface of the body.
In fact, this is the power of thermal radiation. The dimension of the radiation flux is [J / s \u003d W].
Energy luminosity (Re) - energy of thermal radiation emitted per unit time from a unit surface of a heated body:
In the SI system, energy luminosity is measured - [W / m 2].
The radiation flux and energy luminosity depend on the structure of the substance and its temperature: F = F(T),
The distribution of energy luminosity over the spectrum of thermal radiation characterizes its spectral density. Let us denote the energy of thermal radiation emitted by a single surface in 1 s in a narrow range of wavelengths from λ before λ +d λ, via dRe.
Spectral density of energy luminosity(r) or emissivity is the ratio of the energy luminosity in a narrow part of the spectrum (dRe) to the width of this part (dλ):
An approximate view of the spectral density and energy luminosity (dRe) in the wavelength range from λ before λ +d λ, shown in fig. 13.1.
Rice. 13.1. Spectral density of energy luminosity
The dependence of the spectral density of energy luminosity on the wavelength is called body radiation spectrum. Knowing this dependence makes it possible to calculate the energy luminosity of a body in any wavelength range. The formula for calculating the energy luminosity of a body in the wavelength range is:
The total luminosity is:
Bodies not only emit, but also absorb thermal radiation. The ability of a body to absorb radiation energy depends on its substance, temperature, and radiation wavelength. The absorption capacity of the body is characterized by monochromatic absorption coefficientα.
Let a stream fall on the surface of the body monochromatic radiation Φ λ with wavelength λ. Part of this flow is reflected and part is absorbed by the body. Let us denote the value of the absorbed flux Φ λ abs.
Monochromatic absorption coefficient α λ is the ratio of the radiation flux absorbed by a given body to the magnitude of the incident monochromatic flux:
The monochromatic absorption coefficient is a dimensionless quantity. Its values lie between zero and one: 0 ≤ α ≤ 1.
Function α = α(λ,Τ) , expressing the dependence of the monochromatic absorption coefficient on the wavelength and temperature, is called absorption capacity body. Her appearance can be quite complex. The simplest types of absorption are considered below.
Completely black body is a body whose absorption coefficient is equal to unity for all wavelengths: α = 1.
gray body is a body for which the absorption coefficient does not depend on the wavelength: α = const< 1.
Absolutely white body is a body whose absorption coefficient is zero for all wavelengths: α = 0.
Kirchhoff's law
Kirchhoff's law- the ratio of the emissivity of the body to its absorption capacity is the same for all bodies and is equal to the spectral density of the energy luminosity of a black body:
= /
Consequence of the law:
1. If a body at a given temperature does not absorb any radiation, then it does not emit it. Indeed, if for some wavelength the absorption coefficient α = 0, then r = α∙ε(λT) = 0
1. At the same temperature black body radiates more than any other. Indeed, for all bodies except black,α < 1, поэтому для них r = α∙ε(λT) < ε
2. If for some body we experimentally determine the dependence of the monochromatic absorption coefficient on the wavelength and temperature - α = r = α(λT), then we can calculate its emission spectrum.
§ 4 Energy luminosity. Stefan-Boltzmann law.
Wien's displacement law
RE(integrated energy luminosity) - energy luminosity determines the amount of energy emitted from a single surface per unit time in the entire frequency range from 0 to ∞ at a given temperature T.
Connection energy luminosity and radiance
[R e ] \u003d J / (m 2 s) \u003d W / m 2
The law of J. Stefan (Austrian scientist) and L. Boltzmann (German scientist)
where
σ \u003d 5.67 10 -8 W / (m 2 K 4) - Stef-on-Boltzmann constant.
The energy luminosity of a black body is proportional to the fourth power of thermodynamic temperature.
Stefan-Boltzmann's law, defining dependenceREon temperature, does not give an answer regarding the spectral composition of the radiation of a completely black body. From the experimental dependence curvesrλ ,T from λ at various T it follows that the distribution of energy in the spectrum of a blackbody is uneven. All curves have a maximum, which with increasing T shifts towards shorter wavelengths. Area bounded by dependency curverλ ,T from λ, is equal to RE(this follows from the geometric meaning of the integral) and is proportional to T 4 .
Wien's displacement law (1864 - 1928): Length, waves (λ max), which accounts for the maximum emissivity of an a.ch.t. at a given temperature, is inversely proportional to the temperature T.
b\u003d 2.9 10 -3 m K - Wien's constant.
The Wien shift occurs because as the temperature increases, the maximum emissivity shifts towards shorter wavelengths.
§ 5 Rayleigh-Jeans formula, Wien's formula and ultraviolet catastrophe
The Stefan-Boltzmann law allows you to determine the energy luminosityREa.h.t. by its temperature. Wien's displacement law relates body temperature to the wavelength at which the maximum emissivity falls. But neither one nor the other law solves the main problem of how great is the radiative ability per each λ in the spectrum of an A.Ch.T. at a temperature T. To do this, you need to establish a functional dependencyrλ ,T from λ and T.
Based on the concept of the continuous nature of the emission of electromagnetic waves in the law of uniform distribution of energies over degrees of freedom, two formulas were obtained for the emissivity of an a.ch.t.:
- Wine Formula
where a, b = const.
- Rayleigh-Jeans formula
k =1.38·10 -23 J/K - Boltzmann's constant.
Experimental verification showed that for a given temperature Wien's formula is correct for short waves and gives sharp discrepancies with experience in the region of long waves. The Rayleigh-Jeans formula turned out to be correct for long waves and not applicable for short ones.
The study of thermal radiation using the Rayleigh-Jeans formula showed that within the framework of classical physics it is impossible to solve the problem of the function characterizing the emissivity of an AChT. This unsuccessful attempt to explain the laws of radiation of A.Ch.T. with the help of the apparatus of classical physics, it was called the “ultraviolet catastrophe”.
If we try to calculateREusing the Rayleigh-Jeans formula, then
- “ ultraviolet catastrophe”
§6 Quantum hypothesis and Planck's formula.
In 1900, M. Planck (a German scientist) put forward a hypothesis according to which the emission and absorption of energy does not occur continuously, but in certain small portions - quanta, and the quantum energy is proportional to the oscillation frequency (Planck's formula):
h \u003d 6.625 10 -34 J s - Planck's constant or
where
Since the radiation occurs in portions, the energy of the oscillator (oscillating atom, electron) E takes only values that are multiples of an integer number of elementary portions of energy, that is, only discrete values
E = n E o = nhν .
The influence of light on the course of electrical processes was first studied by Hertz in 1887. He conducted experiments with an electric spark gap and found that when irradiated with ultraviolet radiation, the discharge occurs at a much lower voltage.
In 1889-1895. A.G. Stoletov studied the effect of light on metals using the following scheme. Two electrodes: cathode K made of the metal under study and anode A (in Stoletov's scheme - a metal mesh that transmits light) in a vacuum tube are connected to the battery so that with the help of resistance R you can change the value and sign of the voltage applied to them. When the zinc cathode was irradiated, a current flowed in the circuit, which was recorded by a milliammeter. By irradiating the cathode with light of various wavelengths, Stoletov established the following basic laws:
- The strongest effect is exerted by ultraviolet radiation;
- Under the action of light, negative charges escape from the cathode;
- The strength of the current generated by the action of light is directly proportional to its intensity.
Lenard and Thomson in 1898 measured the specific charge ( e/ m), ejected particles, and it turned out that it is equal to the specific charge of the electron, therefore, electrons are ejected from the cathode.
§ 2 External photoelectric effect. Three laws of the external photoelectric effect
The external photoelectric effect is the emission of electrons by a substance under the influence of light. Electrons escaping from a substance with an external photoelectric effect are called photoelectrons, and the current they generate is called photocurrent.
Using the Stoletov scheme, the following dependence of the photocurrent onapplied voltage at constant luminous flux F(that is, the I–V characteristic was obtained - current-voltage characteristic):At some voltageUHphotocurrent reaches saturationI n - all electrons emitted by the cathode reach the anode, hence the saturation currentI n is determined by the number of electrons emitted by the cathode per unit time under the action of light. The number of released photoelectrons is proportional to the number of light quanta incident on the cathode surface. And the number of light quanta is determined by the luminous flux F falling on the cathode. Number of photonsNfalling over timet to the surface is determined by the formula:
where W- radiation energy received by the surface during the time Δt,
photon energy,
F e -luminous flux (radiation power).
1st law of external photoelectric effect (Stoletov's law):
At a fixed frequency of the incident light, the saturation photocurrent is proportional to the incident light flux:
Ius~ Ф, ν =const
Uh - retarding voltage is the voltage at which no electron can reach the anode. Therefore, the law of conservation of energy in this case can be written: the energy of the emitted electrons is equal to the retarding energy of the electric field
therefore, one can find the maximum velocity of the emitted photoelectronsVmax
2nd law of photoelectric effect : maximum initial speedVmaxphotoelectrons does not depend on the intensity of the incident light (on F), but is determined only by its frequency ν
3rd law of photoelectric effect : for every substance there is "red border" photo effect, that is, the minimum frequency ν kp , depending on the chemical nature of the substance and the state of its surface, at which the external photoelectric effect is still possible.
The second and third laws of the photoelectric effect cannot be explained using the wave nature of light (or the classical electromagnetic theory of light). According to this theory, the pulling out of conduction electrons from the metal is the result of their "rocking" by the electromagnetic field of the light wave. As the light intensity increases ( F) the energy transmitted by the electron of the metal should increase, therefore, it should increaseVmax, and this contradicts the 2nd law of the photoelectric effect.
Since, according to the wave theory, the energy transmitted by the electromagnetic field is proportional to the intensity of light ( F), then any light; frequency, but a sufficiently high intensity would have to pull out electrons from the metal, that is, the red border of the photoelectric effect would not exist, which contradicts the 3rd law of the photoelectric effect. The external photoelectric effect is inertialess. And the wave theory cannot explain its inertialessness.
§ 3 Einstein's equation for the external photoelectric effect.
Work function
In 1905, A. Einstein explained the photoelectric effect on the basis of quantum concepts. According to Einstein, light is not only emitted by quanta in accordance with Planck's hypothesis, but propagates in space and is absorbed by matter in separate portions - quanta with energy E0 = hv. The quanta of electromagnetic radiation are called photons.
Einstein's equation (the law of conservation of energy for the external photo effect):
Incident photon energy hv is spent on pulling out an electron from the metal, that is, on the work function A out, and to communicate kinetic energy to the emitted photoelectron.
The smallest energy that must be imparted to an electron in order to remove it from a solid body into a vacuum is called work function.
Since the energy of Ferm to E Fdepends on temperature and E F, also changes with temperature, then, therefore, A out temperature dependent.
In addition, the work function is very sensitive to surface finish. Applying a film to the surface Sa, SG, Wa) on the WA outdecreases from 4.5 eV for pureW up to 1.5 h 2 eV for impurityW.
Einstein's equation makes it possible to explain in c e three laws of the external photo-effect,
1st law: each quantum is absorbed by only one electron. Therefore, the number of ejected photoelectrons should be proportional to the intensity ( F) Sveta
2nd law: Vmax~ ν and since A out does not depend on F, then andVmax does not depend on F
3rd law: As ν decreases,Vmax and for ν = ν 0 Vmax = 0, therefore,hν 0 = A out, therefore, i.e. there is a minimum frequency, starting from which the external photoelectric effect is possible.
d Φ e (\displaystyle d\Phi _(e)), emitted by a small area of the surface of the radiation source, to its area d S (\displaystyle dS) : M e = d Φ e d S . (\displaystyle M_(e)=(\frac (d\Phi _(e))(dS)).)They also say that the energy luminosity is the surface density of the emitted radiation flux.
Numerically, the energy luminosity is equal to the time-averaged modulus of the component of the Poynting vector perpendicular to the surface. In this case, averaging is carried out over a time that significantly exceeds the period of electromagnetic oscillations.
The emitted radiation can originate in the surface itself, then one speaks of a self-luminous surface. Another variant is observed when the surface is illuminated from outside. In such cases, some part of the incident flux necessarily returns as a result of scattering and reflection. Then the expression for the energy luminosity has the form:
M e = (ρ + σ) ⋅ E e , (\displaystyle M_(e)=(\rho +\sigma)\cdot E_(e),)where ρ (\displaystyle \rho ) and σ (\displaystyle \sigma )- coefficient reflection and coefficient scattering of the surface, respectively, and - its irradiance .
Other names of energy luminosity, sometimes used in the literature, but not provided for by GOST: - emissivity and integral emissivity.
Spectral density of energy luminosity
Spectral density of energy luminosity M e , λ (λ) (\displaystyle M_(e,\lambda )(\lambda))- the ratio of the magnitude of the energy luminosity d M e (λ) , (\displaystyle dM_(e)(\lambda),) per small spectral interval d λ , (\displaystyle d\lambda ,) enclosed between λ (\displaystyle \lambda ) and λ + d λ (\displaystyle \lambda +d\lambda ), to the width of this interval:
M e , λ (λ) = d M e (λ) d λ . (\displaystyle M_(e,\lambda )(\lambda)=(\frac (dM_(e)(\lambda))(d\lambda )).)The SI unit of measure is W m −3 . Since the wavelengths of optical radiation are usually measured in nanometers, in practice W m −2 nm −1 is often used.
Sometimes in literature M e , λ (\displaystyle M_(e,\lambda )) are called spectral emissivity.
Light analogue
M v = K m ⋅ ∫ 380 n m 780 n m M e , λ (λ) V (λ) d λ , (\displaystyle M_(v)=K_(m)\cdot \int \limits _(380~nm)^ (780~nm)M_(e,\lambda )(\lambda)V(\lambda)d\lambda ,)where K m (\displaystyle K_(m))- maximum luminous efficiency of radiation, equal in the SI system to 683 lm / W. Its numerical value follows directly from the definition of the candela.
Information about other basic energy photometric quantities and their light analogs is given in the table. The designations of the quantities are given according to GOST 26148-84.
SI energy photometric quantities| Name (synonym) | Value designation | Definition | SI unit notation | Light value |
|---|---|---|---|---|
| Energy radiation (radiant energy) | Q e (\displaystyle Q_(e)) or W (\displaystyle W) | Energy carried by radiation | J | Light energy |
| Flux radiation (radiant flux) | Φ (\displaystyle \Phi ) or P (\displaystyle P) | Φ e = d Q e d t (\displaystyle \Phi _(e)=(\frac (dQ_(e))(dt))) | Tue | Light flow |
| Strength radiation (energy strength of light) | I e (\displaystyle I_(e)) | I e = d Φ e d Ω (\displaystyle I_(e)=(\frac (d\Phi _(e))(d\Omega ))) | Tue sr −1 | The power of light |
| Volumetric radiation energy density | U e (\displaystyle U_(e)) | U e = d Q e d V (\displaystyle U_(e)=(\frac (dQ_(e))(dV))) | J m −3 | Volumetric density of light energy |
| Energy brightness | L e (\displaystyle L_(e)) | L e = d 2 Φ e d Ω d S 1 cos ε (\displaystyle L_(e)=(\frac (d^(2)\Phi _(e))(d\Omega \,dS_(1)\, \cos\varepsilon))) | W m −2 sr −1 | Brightness |
| Integral energy brightness | e (\displaystyle \Lambda _(e)) | Λ e = ∫ 0 t L e (t ′) d t ′ (\displaystyle \Lambda _(e)=\int _(0)^(t)L_(e)(t")dt") | J m −2 sr −1 | Integral brightness |
| Irradiation (energy illumination) | E e (\displaystyle E_(e)) | E e = d Φ e d S 2 (\displaystyle E_(e)=(\frac (d\Phi _(e))(dS_(2)))) | W m −2 |
1. Characteristics of thermal radiation.
2. Kirchhoff's law.
3. Laws of radiation of a black body.
4. Radiation of the Sun.
5. Physical foundations of thermography.
6. Light therapy. Therapeutic uses of ultraviolet light.
7. Basic concepts and formulas.
8. Tasks.
From the whole variety of electromagnetic radiation, visible or invisible to the human eye, one can be distinguished, which is inherent in all bodies - this is thermal radiation.
thermal radiation- electromagnetic radiation emitted by a substance and arising due to its internal energy.
Thermal radiation is caused by the excitation of particles of matter during collisions in the process of thermal motion or by the accelerated motion of charges (oscillations of crystal lattice ions, thermal motion of free electrons, etc.). It occurs at any temperature and is inherent in all bodies. A characteristic feature of thermal radiation is continuous spectrum.
The intensity of radiation and the spectral composition depend on body temperature, therefore, thermal radiation is not always perceived by the eye as a glow. For example, bodies heated to a high temperature emit a significant part of the energy in the visible range, and at room temperature almost all of the energy is emitted in the infrared part of the spectrum.
26.1. Characteristics of thermal radiation
The energy that a body loses due to thermal radiation is characterized by the following values.
radiation flux(F) - energy radiated per unit time from the entire surface of the body.
In fact, this is the power of thermal radiation. The dimension of the radiation flux is [J / s \u003d W].
Energy luminosity(Re) is the energy of thermal radiation emitted per unit time from a unit surface of a heated body:
The dimension of this characteristic is [W / m 2].
Both the radiation flux and the energy luminosity depend on the structure of the substance and its temperature: Ф = Ф(Т), Re = Re(T).
The distribution of energy luminosity over the spectrum of thermal radiation characterizes its spectral density. Let us denote the energy of thermal radiation emitted by a single surface in 1 s in a narrow range of wavelengths from λ before λ +d λ, via dRe.
The spectral density of energy luminosity(r) or emissivity is the ratio of energy luminosity in a narrow part of the spectrum (dRe) to the width of this part (dλ):
An approximate view of the spectral density and energy luminosity (dRe) in the wavelength range from λ before λ +d λ, shown in fig. 26.1.
Rice. 26.1. Spectral density of energy luminosity
The dependence of the spectral density of energy luminosity on the wavelength is called body radiation spectrum. Knowing this dependence allows you to calculate the energy luminosity of the body in any wavelength range:
Bodies not only emit, but also absorb thermal radiation. The ability of a body to absorb radiation energy depends on its substance, temperature, and radiation wavelength. The absorption capacity of the body is characterized by monochromatic absorption coefficientα.
Let a stream fall on the surface of the body monochromatic radiation Φ λ with wavelength λ. Part of this flow is reflected and part is absorbed by the body. Let us denote the value of the absorbed flux Φ λ abs.
Monochromatic absorption coefficient α λ is the ratio of the radiation flux absorbed by a given body to the magnitude of the incident monochromatic flux:
The monochromatic absorption coefficient is a dimensionless quantity. Its values lie between zero and one: 0 ≤ α ≤ 1.
The function α = α(λ,Τ), which expresses the dependence of the monochromatic absorption coefficient on the wavelength and temperature, is called absorption capacity body. Her appearance can be quite complex. The simplest types of absorption are considered below.
Completely black body- such a body, the absorption coefficient of which is equal to unity for all wavelengths: α \u003d 1. It absorbs all the radiation incident on it.
According to their absorption properties, soot, black velvet, platinum black are close to an absolutely black body. A very good model of a blackbody is a closed cavity with a small hole (O). The walls of the cavity are blackened in Fig. 26.2.
The beam entering this hole is almost completely absorbed after multiple reflections from the walls. Similar devices
Rice. 26.2. Black body model
used as light standards, used in measuring high temperatures, etc.
The spectral density of the energy luminosity of a completely black body is denoted by ε(λ, Τ). This function plays an important role in the theory of thermal radiation. Its form was first established experimentally, and then obtained theoretically (Planck's formula).
Absolutely white body- such a body, the absorption coefficient of which is equal to zero for all wavelengths: α = 0.
There are no truly white bodies in nature, however, there are bodies that are close to them in properties in a fairly wide range of temperatures and wavelengths. For example, a mirror in the optical part of the spectrum reflects almost all the incident light.
gray body is a body for which the absorption coefficient does not depend on the wavelength: α = const< 1.
Some real bodies have this property in a certain range of wavelengths and temperatures. For example, "gray" (α = 0.9) can be considered human skin in the infrared region.
26.2. Kirchhoff's law
The quantitative relationship between radiation and absorption was established by G. Kirchhoff (1859).
Kirchhoff's law- attitude emissivity body to his absorption capacity the same for all bodies and equal to the spectral density of the energy luminosity of a completely black body:
We note some consequences of this law.
1. If a body at a given temperature does not absorb any radiation, then it does not emit it. Indeed, if for
26.3. Laws of blackbody radiation
The laws of black body radiation were established in the following sequence.
In 1879, J. Stefan experimentally, and in 1884, L. Boltzmann theoretically determined energy luminosity absolutely black body.
Stefan-Boltzmann law - The energy luminosity of a blackbody is proportional to the fourth power of its absolute temperature:
The values of the absorption coefficients for some materials are given in Table. 26.1.
Table 26.1. absorption coefficients
The German physicist W. Wien (1893) established a formula for the wavelength that accounts for the maximum emissivity absolutely black body. The ratio he received was named after him.
As the temperature rises, the maximum emissivity is shifting to the left (Fig. 26.3).
Rice. 26.3. Wien's displacement law illustration
In table. 26.2 shows the colors in the visible part of the spectrum, corresponding to the radiation of bodies at different temperatures.
Table 26.2. Colors of heated bodies
Using the laws of Stefan-Boltzmann and Wien, it is possible to determine the temperatures of bodies by measuring the radiation of these bodies. For example, the temperature of the surface of the Sun (~6000 K), the temperature at the epicenter of the explosion (~10 6 K), etc. are determined in this way. The common name for these methods is pyrometry.
In 1900, M. Planck received a formula for calculating emissivity absolutely black body theoretically. To do this, he had to abandon the classical ideas about continuity the process of radiation of electromagnetic waves. According to Planck, the radiation flux consists of separate portions - quanta, whose energies are proportional to the frequencies of light:
From formula (26.11) one can theoretically obtain the laws of Stefan-Boltzmann and Wien.
26.4. Sun radiation
Within the solar system, the Sun is the most powerful source of thermal radiation that determines life on Earth. Solar radiation has healing properties (heliotherapy), is used as a means of hardening. It can also have a negative effect on the body (burn, thermal
The spectra of solar radiation at the boundary of the earth's atmosphere and at the surface of the earth are different (Fig. 26.4).
Rice. 26.4. Spectrum of solar radiation: 1 - at the boundary of the atmosphere, 2 - at the surface of the Earth
At the boundary of the atmosphere, the spectrum of the Sun is close to the spectrum of a black body. The maximum emissivity is at λ1max= 470 nm (blue).
Near the Earth's surface, the spectrum of solar radiation has a more complex shape, which is associated with absorption in the atmosphere. In particular, it does not contain the high-frequency part of ultraviolet radiation, which is harmful to living organisms. These rays are almost completely absorbed by the ozone layer. The maximum emissivity is at λ2max= 555 nm (green-yellow), which corresponds to the best eye sensitivity.
The flux of solar thermal radiation at the boundary of the earth's atmosphere determines solar constant I.
The flux reaching the earth's surface is much less due to absorption in the atmosphere. Under the most favorable conditions (the sun at its zenith), it does not exceed 1120 W / m 2. In Moscow at the time of the summer solstice (June) - 930 W / m 2.
Both the power of solar radiation near the earth's surface and its spectral composition depend most significantly on the height of the Sun above the horizon. On fig. 26.5 the smoothed curves of distribution of energy of a sunlight are given: I - outside of an atmosphere; II - at the position of the Sun at the zenith; III - at a height of 30 ° above the horizon; IV - under conditions close to sunrise and sunset (10° above the horizon).
Rice. 26.5. Distribution of energy in the spectrum of the Sun at different heights above the horizon
Different components of the solar spectrum pass through the Earth's atmosphere in different ways. Figure 26.6 shows the transparency of the atmosphere at high altitudes of the Sun.
26.5. Physical basis of thermography
The thermal radiation of a person makes up a significant proportion of his thermal losses. The radiative loss of a person is equal to the difference emitted flow and absorbed environmental radiation flux. Radiative loss power is calculated by the formula
where S is the surface area; δ - reduced absorption coefficient of the skin (clothing), considered as gray body; T 1 - body surface temperature (clothes); T 0 - ambient temperature.
Consider the following example.
Let us calculate the power of radiative losses of a naked person at an ambient temperature of 18°C (291 K). Let's take: the surface area of the body S = 1.5 m 2; skin temperature T 1 = 306 K (33°C). The reduced absorption coefficient of the skin is found in Table. 26.1 (δ \u003d 5.1 * 10 -8 W / m 2 K 4). Substituting these values into formula (26.11), we obtain
P \u003d 1.5 * 5.1 * 10 -8 * (306 4 - 291 4) ≈122 W.
Rice. 26.6. The transparency of the earth's atmosphere (in percent) for different parts of the spectrum at a high altitude of the Sun.
Human thermal radiation can be used as a diagnostic parameter.
Thermography - a diagnostic method based on the measurement and registration of thermal radiation from the surface of the human body or its individual sections.
The temperature distribution over a small area of the body surface can be determined using special liquid crystal films. Such films are sensitive to small temperature changes (change color). Therefore, a color thermal “portrait” of the body area on which it is superimposed appears on the film.
A more advanced way is to use thermal imagers that convert infrared radiation into visible light. The radiation of the body is projected onto the matrix of the thermal imager using a special lens. After conversion, a detailed thermal portrait is formed on the screen. Areas with different temperatures differ in color or intensity. Modern methods allow fixing the difference in temperatures up to 0.2 degrees.
Thermal portraits are used in functional diagnostics. Various pathologies of the internal organs can form on the surface skin zones with a changed temperature. The detection of such zones indicates the presence of pathology. The thermographic method facilitates the differential diagnosis between benign and malignant tumors. This method is an objective means of monitoring the effectiveness of therapeutic methods of treatment. So, during a thermographic examination of patients with psoriasis, it was found that in the presence of severe infiltration and hyperemia in plaques, an increase in temperature is noted. A decrease in temperature to the level of the surrounding areas in most cases indicates regression process on the skin.
Fever is often an indicator of infection. To determine the temperature of a person, it is enough to look through an infrared device at his face and neck. For healthy people, the ratio of forehead temperature to carotid temperature ranges from 0.98 to 1.03. This ratio can be used in express diagnostics during epidemics for quarantine measures.
26.6. Phototherapy. Therapeutic uses of ultraviolet light
Infrared radiation, visible light and ultraviolet radiation are widely used in medicine. Recall the ranges of their wavelengths:
Phototherapy called the use of infrared and visible radiation for therapeutic purposes.
Penetrating into tissues, infrared rays (as well as visible ones) in the place of their absorption cause the release of heat. The depth of penetration of infrared and visible rays into the skin is shown in Fig. 26.7.
Rice. 26.7. Depth of radiation penetration into the skin
In medical practice, special irradiators are used as sources of infrared radiation (Fig. 26.8).
Minin lamp is an incandescent lamp with a reflector that localizes the radiation in the required direction. The radiation source is a 20-60 W incandescent lamp made of colorless or blue glass.
Light-thermal bath is a semi-cylindrical frame, consisting of two halves connected movably to each other. On the inner surface of the frame, facing the patient, incandescent lamps with a power of 40 W are fixed. In such baths, the biological object is affected by infrared and visible radiation, as well as heated air, the temperature of which can reach 70°C.
Lamp Sollux is a powerful incandescent lamp placed in a special reflector on a tripod. The source of radiation is an incandescent lamp with a power of 500 W (tungsten filament temperature 2800°C, radiation maximum at a wavelength of 2 μm).
Rice. 26.8. Irradiators: Minin lamp (a), light-thermal bath (b), Sollux lamp (c)
Therapeutic uses of ultraviolet light
Ultraviolet radiation used for medical purposes is divided into three ranges:
When ultraviolet radiation is absorbed in tissues (in the skin), various photochemical and photobiological reactions occur.
used as radiation sources. high pressure lamps(arc, mercury, tubular), fluorescent lamps, gas discharge low pressure lamps one of the varieties of which are bactericidal lamps.
A radiation has an erythemal and tanning effect. It is used in the treatment of many dermatological diseases. Some chemical compounds of the furocoumarin series (for example, psoralen) are able to sensitize the skin of these patients to long-wave ultraviolet radiation and stimulate the formation of melanin pigment in melanocytes. The combined use of these drugs with A-radiation is the basis of a treatment method called photochemotherapy or PUVA therapy(PUVA: P - psoralen; UVA - ultraviolet radiation zone A). Part or all of the body is exposed to radiation.
B radiation has a vitamin-forming, anti-rachitic effect.
C radiation has a bactericidal effect. Irradiation destroys the structure of microorganisms and fungi. C-radiation is created by special bactericidal lamps (Fig. 26.9).
Some medical techniques use C-radiation to irradiate the blood.
Ultraviolet starvation. Ultraviolet radiation is necessary for the normal development and functioning of the body. Its deficiency leads to a number of serious diseases. Residents of the extreme region face ultraviolet starvation
Rice. 26.9. Bactericidal irradiator (a), nasopharyngeal irradiator (b)
North, workers in the mining industry, the subway, residents of large cities. In cities, the lack of ultraviolet radiation is associated with air pollution by dust, smoke, and gases that block the UV part of the solar spectrum. The windows of the premises do not transmit UV rays with a wavelength of λ< 310 нм. Значительно снижают УФ-поток загрязненные стекла и занавеси (тюлевые занавески снижают УФ-излучение на 20 %). Поэтому на многих производствах и в быту наблюдается так называемая «биологическая полутьма». В первую очередь страдают дети (возрастает вероятность заболевания рахитом).
The dangers of ultraviolet radiation
Exposure to excess doses of ultraviolet radiation on the body as a whole and on its individual organs leads to a number of pathologies. First of all, this refers to the consequences of uncontrolled sunbathing: burns, age spots, eye damage - the development of photophthalmia. The effect of ultraviolet radiation on the eye is similar to erythema, since it is associated with the decomposition of proteins in the cells of the cornea and mucous membranes of the eye. Living human skin cells are protected from the destructive action of UV rays "dead-
mi" cells of the stratum corneum of the skin. The eyes are deprived of this protection, therefore, with a significant dose of eye irradiation, inflammation of the horny (keratitis) and mucous membranes (conjunctivitis) of the eye develops after a latent period. This effect is due to rays with a wavelength less than 310 nm. It is necessary to protect the eye from such rays. Special attention should be paid to the blastomogenic effect of UV radiation, leading to the development of skin cancer.
26.7. Basic concepts and formulas
Table continuation
End of table
26.8. Tasks
2. Determine how many times the energy luminosities of areas of the surface of the human body differ, having temperatures of 34 and 33 ° C, respectively?
3. When diagnosing a breast tumor by thermography, the patient is given a glucose solution to drink. After some time, the thermal radiation of the body surface is recorded. Tumor tissue cells intensively absorb glucose, as a result of which their heat production increases. By how many degrees does the temperature of the skin area above the tumor change if the radiation from the surface increases by 1% (1.01 times)? The initial temperature of the body area is 37°C.
6. How much did the human body temperature increase if the radiation flux from the body surface increased by 4%? The initial body temperature is 35°C.
7. There are two identical kettles in a room containing equal masses of water at 90°C. One is nickel plated and the other is black. Which kettle will cool the fastest? Why?
Solution
According to Kirchhoff's law, the ratio of emitting and absorbing abilities is the same for all bodies. Nickel-plated teapot reflects almost all light. Therefore, its absorption capacity is small. Accordingly, the emissivity is also small.
Answer: the dark kettle will cool faster.
8. For the destruction of pest bugs, the grain is exposed to infrared radiation. Why do the bugs die, but the grain does not?
Answer: bugs have black color, therefore intensively absorb infrared radiation and perish.
9. When heating a piece of steel, we will observe a bright cherry-red heat at a temperature of 800 ° C, but a transparent rod of fused quartz does not glow at all at the same temperature. Why?
Solution
See problem 7. A transparent body absorbs a small part of the light. Therefore, its emissivity is small.
Answer: a transparent body practically does not radiate, even when it is strongly heated.
10. Why do many animals sleep curled up in cold weather?
Answer: in this case, the open surface of the body decreases and, accordingly, the radiation losses decrease.
Thermal radiation of bodies is called electromagnetic radiation that occurs due to that part of the internal energy of the body, which is related to the thermal motion of its particles.
The main characteristics of thermal radiation of bodies heated to a temperature T are:
1. Energy luminosityR (T ) -the amount of energy emitted per unit time per unit surface of the body, over the entire wavelength range. Depends on the temperature, nature and state of the surface of the radiating body. In the SI system R ( T ) has the dimension [W/m 2 ].
2. Spectral density of energy luminosityr ( ,T) =dW/ d - the amount of energy emitted by a unit of body surface per unit of time in a unit wavelength interval (near the considered wavelength ). Those. this quantity is numerically equal to the energy ratio dW emitted per unit area per unit time in a narrow range of wavelengths from before +d , to the width of this interval. It depends on the temperature of the body, the wavelength, and also on the nature and condition of the surface of the radiating body. In the SI system r( , T) has the dimension [W/m 3 ].
Energy luminosity R(T) related to the spectral density of energy luminosity r( , T) in the following way:
(1) [W/m2]
3. All bodies not only radiate, but also absorb electromagnetic waves incident on their surface. To determine the absorption capacity of bodies in relation to electromagnetic waves of a certain wavelength, the concept is introduced monochromatic absorption coefficient-the ratio of the energy of a monochromatic wave absorbed by the body surface to the energy of an incident monochromatic wave:
The monochromatic absorption coefficient is a dimensionless quantity that depends on temperature and wavelength. It shows what fraction of the energy of the incident monochromatic wave is absorbed by the surface of the body. Value ( , T) can take values from 0 to 1.
Radiation in an adiabatically closed system (not exchanging heat with the environment) is called equilibrium. If a small hole is created in the wall of the cavity, the state of equilibrium will change slightly, and the radiation leaving the cavity will correspond to the equilibrium radiation.
If a beam is directed into such a hole, then after repeated reflections and absorption on the walls of the cavity, it will not be able to go back out. This means that for such a hole, the absorption coefficient ( , T) = 1.
The considered closed cavity with a small hole serves as one of the models absolutely black body.
Completely black bodya body is called that absorbs all the radiation incident on it, regardless of the direction of the incident radiation, its spectral composition and polarization (without reflecting or transmitting anything).
For a blackbody, the spectral density of energy luminosity is some universal function of wavelength and temperature f( , T) and does not depend on its nature.
All bodies in nature partially reflect the radiation incident on their surface and therefore do not belong to absolutely black bodies. If the monochromatic absorption coefficient of a body is the same for all wavelengths and lessunits(( , T) = Т = const<1),then such a body is called gray. The coefficient of monochromatic absorption of a gray body depends only on the temperature of the body, its nature and the state of its surface.
Kirchhoff showed that for all bodies, regardless of their nature, the ratio of the spectral density of energy luminosity to the monochromatic absorption coefficient is the same universal function of wavelength and temperature f( , T) , which is the spectral density of the energy luminosity of a black body :
Equation (3) is Kirchhoff's law.
Kirchhoff's law can be formulated like this: for all bodies of the system that are in thermodynamic equilibrium, the ratio of the spectral density of energy luminosity to the coefficient monochromatic absorption does not depend on the nature of the body, is the same function for all bodies, depending on the wavelength and temperature T.
From the foregoing and formula (3) it is clear that at a given temperature, those gray bodies that have a large absorption coefficient radiate more strongly, and absolutely black bodies radiate most strongly. Since for a completely black body( , T)=1, then formula (3) implies that the universal function f( , T) is the spectral density of the energy luminosity of a black body
