Absolutely black body. Absolutely black body - a problem of Newtonian physics

A completely black body that completely absorbs electromagnetic radiation of any frequency, when heated, emits energy in the form of waves evenly distributed over the entire frequency spectrum

By the end of the 19th century, scientists, studying the interaction of electromagnetic radiation (in particular, light) with atoms of matter, encountered serious problems that could only be solved within the framework of quantum mechanics, which, in many ways, arose due to the fact that these problems arose. To understand the first and perhaps most serious of these problems, imagine a large black box with a mirrored interior surface, and in one of the walls there is a small hole made. A ray of light penetrating into a box through a microscopic hole remains inside forever, endlessly reflecting off the walls. An object that does not reflect light, but completely absorbs it, appears black, which is why it is commonly called a black body. (A black body, like many other conceptual physical phenomena, is a purely hypothetical object, although, for example, a hollow, uniformly heated sphere mirrored from the inside, into which light penetrates through a single tiny hole, is a good approximation.)

The model of such a black box helps us understand how the light absorbed by a black body behaves, interacting with the atoms of its substance. Here it is important to understand that light is absorbed by an atom, immediately emitted by it and absorbed by another atom, again emitted and absorbed, and this will happen until the state of equilibrium saturation is reached. When a black body is heated to an equilibrium state, the intensities of emission and absorption of rays inside the black body are equalized: when a certain amount of light of a certain frequency is absorbed by one atom, another atom somewhere inside simultaneously emits the same amount of light of the same frequency. Thus, the amount of absorbed light of each frequency within a black body remains the same, although different atoms of the body absorb and emit it.

Until this moment, the behavior of the black body remains quite understandable. Problems within the framework of classical physics (by “classical” here we mean physics before the advent of quantum mechanics) began when trying to calculate the radiation energy stored inside a black body in an equilibrium state. And two things soon became clear:

1. the higher the wave frequency of the rays, the more of them accumulate inside the black body (that is, the shorter the wavelengths of the studied part of the spectrum of radiation waves, the more rays of this part of the spectrum inside the black body are predicted by the classical theory);
2. The higher the frequency of the wave, the more energy it carries and, accordingly, the more of it is stored inside the black body.

The German physicist Max Planck managed to restore order (see Planck's constant) - he showed that the problem is removed if we assume that atoms can absorb and emit light only in portions and only at certain frequencies. (Later, Albert Einstein generalized this idea by introducing the concept of photons - strictly defined portions of light radiation.) According to this scheme, many frequencies of radiation predicted by classical physics simply cannot exist inside a black body, since atoms are unable to absorb or emit them; Accordingly, these frequencies are excluded from consideration when calculating the equilibrium radiation inside a black body. By leaving only permissible frequencies, Planck prevented the ultraviolet catastrophe and set science on the path to a correct understanding of the structure of the world at the subatomic level. In addition, he calculated the characteristic frequency distribution of equilibrium black body radiation.

This distribution gained worldwide fame many decades after its publication by Planck himself, when cosmologists discovered that the cosmic microwave background radiation they discovered exactly obeys the Planck distribution in its spectral characteristics and corresponds to the radiation of a completely black body at a temperature of about three degrees above absolute zero.

Encyclopedia by James Trefil “The Nature of Science. 200 laws of the universe."
James Trefil is a professor of physics at George Mason University (USA), one of the most famous Western authors of popular science books.

One of the facts of the subatomic world is that its objects - such as electrons or photons - are not at all similar to the usual objects of the macroworld. They behave neither like particles nor like waves, but like completely special formations that exhibit both wave and corpuscular properties depending on the circumstances. It is one thing to make a statement, but quite another to connect together the wave and particle aspects of the behavior of quantum particles, describing them with an exact equation. This is exactly what was done in the de Broglie relation.

In everyday life, there are two ways to transfer energy in space - through particles or waves. In everyday life, there are no visible contradictions between the two mechanisms of energy transfer. So, a basketball is a particle, and sound is a wave, and everything is clear. However, in quantum mechanics things are not so simple. Even from the simplest experiments with quantum objects, it very soon becomes clear that in the microworld the principles and laws of the macroworld that we are familiar with do not apply. Light, which we are accustomed to thinking of as a wave, sometimes behaves as if it consists of a stream of particles (photons), and elementary particles, such as an electron or even a massive proton, often exhibit the properties of a wave.

There are a number of types of electromagnetic radiation, ranging from radio waves to gamma rays. Electromagnetic rays of all types propagate in a vacuum at the speed of light and differ from each other only in wavelengths.

The dual particle-wave nature of quantum particles is described by a differential equation.

Max Planck, one of the founders of quantum mechanics, came to the ideas of energy quantization, trying to theoretically explain the process of interaction between recently discovered electromagnetic waves and atoms and, thereby, solve the problem of black body radiation. He realized that to explain the observed emission spectrum of atoms, it is necessary to take for granted that atoms emit and absorb energy in portions (which the scientist called quanta) and only at individual wave frequencies.

The word “quantum” comes from the Latin quantum (“how much, how much”) and the English quantum (“quantity, portion, quantum”). “Mechanics” has long been the name given to the science of the movement of matter. Accordingly, the term “quantum mechanics” means the science of the movement of matter in portions (or, in modern scientific language, the science of the movement of quantized matter). The term “quantum” was coined by the German physicist Max Planck to describe the interaction of light with atoms.

Most of all, Einstein protested against the need to describe the phenomena of the microworld in terms of probabilities and wave functions, and not from the usual position of coordinates and particle velocities. That's what he meant by "rolling the dice." He recognized that describing the movement of electrons in terms of their speeds and coordinates contradicts the uncertainty principle. But, Einstein argued, there must be some other variables or parameters, taking into account which the quantum mechanical picture of the microworld will return to the path of integrity and determinism. That is, he insisted, it only seems to us that God is playing dice with us, because we do not understand everything. Thus, he was the first to formulate the hidden variable hypothesis in the equations of quantum mechanics. It lies in the fact that in fact electrons have fixed coordinates and speed, like Newton’s billiard balls, and the uncertainty principle and the probabilistic approach to their determination within the framework of quantum mechanics are the result of the incompleteness of the theory itself, which is why it does not allow them for certain define.

Light is the basis of life on our planet. Answering the questions “Why is the sky blue?” and “Why is the grass green?” you can give a definite answer - “Thanks to the light.” This is an integral part of our life, but we are still trying to understand the phenomenon of light...

Waves are one of two ways of energy transfer in space (the other way is corpuscular, using particles). Waves usually propagate in some medium (for example, waves on the surface of a lake propagate in water), but the direction of movement of the medium itself does not coincide with the direction of movement of the waves. Imagine a float bobbing on the waves. Rising and falling, the float follows the movements of the water as the waves pass by it. The phenomenon of interference occurs when two or more waves of the same frequency, propagating in different directions, interact.

The basics of the phenomenon of diffraction can be understood by referring to Huygens' principle, according to which each point along the path of propagation of a light beam can be considered as a new independent source of secondary waves, and the further diffraction pattern is determined by the interference of these secondary waves. When a light wave interacts with an obstacle, some of the secondary Huygens waves are blocked.

By the end of the 19th century, scientists, studying the interaction of electromagnetic radiation (in particular, light) with atoms of matter, encountered serious problems that could only be solved within the framework of quantum mechanics, which, in many ways, arose due to the fact that these problems arose. To understand the first and perhaps most serious of these problems, imagine a large black box with a mirrored interior surface, and in one of the walls there is a small hole made. A ray of light penetrating into a box through a microscopic hole remains inside forever, endlessly reflecting off the walls. An object that does not reflect light, but completely absorbs it, appears black, which is why it is usually called black body. (A black body, like many other conceptual physical phenomena, is a purely hypothetical object, although, for example, a hollow, uniformly heated sphere mirrored from the inside, into which light penetrates through a single tiny hole, is a good approximation.)

You, however, have probably seen quite close analogues of a black body in reality. In a fireplace, for example, it happens that several logs are stacked almost tightly together, and a rather large cavity burns out inside them. The outside of the logs remains dark and does not glow, while inside the burnt cavity heat (infrared radiation) and light accumulate, and these rays are reflected repeatedly from the walls of the cavity before escaping outside. If you look into the gap between such logs, you will see a bright yellow-orange high-temperature glow and from there you will literally be blazing with heat. The rays were simply trapped for some time between the logs, just as light is completely trapped and absorbed by the black box described above.

The model of such a black box helps us understand how the light absorbed by a black body behaves, interacting with the atoms of its substance. Here it is important to understand that light is absorbed by an atom, immediately emitted by it and absorbed by another atom, again emitted and absorbed, and this will happen until the state of equilibrium saturation is reached. When a black body is heated to an equilibrium state, the intensities of emission and absorption of rays inside the black body are equalized: when a certain amount of light of a certain frequency is absorbed by one atom, another atom somewhere inside simultaneously emits the same amount of light of the same frequency. Thus, the amount of absorbed light of each frequency within a black body remains the same, although different atoms of the body absorb and emit it.

Until this moment, the behavior of the black body remains quite understandable. Problems within the framework of classical physics (by “classical” here we mean physics before the advent of quantum mechanics) began when trying to calculate the radiation energy stored inside a black body in an equilibrium state. And two things soon became clear:

• the higher the wave frequency of the rays, the more of them accumulate inside the black body (that is, the shorter the wavelengths of the studied part of the spectrum of radiation waves, the more rays of this part of the spectrum inside the black body are predicted by the classical theory);
• The higher the frequency of the wave, the more energy it carries and, accordingly, the more of it is stored inside the black body.

Taken together, these two conclusions led to an unthinkable result: the radiation energy inside a black body should be infinite! This evil mockery of the laws of classical physics was dubbed ultraviolet disaster, since high-frequency radiation lies in the ultraviolet part of the spectrum.

Order was restored by the German physicist Max Planck ( cm. Planck's constant) - he showed that the problem is removed if we assume that atoms can absorb and emit light only in portions and only at certain frequencies. (Albert Einstein later generalized this idea by introducing the concept photons- strictly defined portions of light radiation.) According to this scheme, many radiation frequencies predicted by classical physics simply cannot exist inside a black body, since atoms are unable to absorb or emit them; Accordingly, these frequencies are excluded from consideration when calculating the equilibrium radiation inside a black body. By leaving only permissible frequencies, Planck prevented the ultraviolet catastrophe and set science on the path to a correct understanding of the structure of the world at the subatomic level. In addition, he calculated the characteristic frequency distribution of equilibrium black body radiation.

This distribution gained worldwide fame many decades after its publication by Planck himself, when cosmologists discovered that the cosmic microwave background radiation they discovered ( cm. The Big Bang) follows exactly the Planck distribution in its spectral characteristics and corresponds to black body radiation at a temperature of about three degrees above absolute zero.

The spectral density of blackbody radiation is a universal function of wavelength and temperature. This means that the spectral composition and radiation energy of a completely black body do not depend on the nature of the body.

Formulas (1.1) and (1.2) show that knowing the spectral and integral radiation density of an absolutely black body, they can be calculated for any non-black body if the absorption coefficient of the latter is known, which must be determined experimentally.

Research led to the following laws of black body radiation.

1. Stefan-Boltzmann law: The integral radiation density of an absolutely black body is proportional to the fourth power of its absolute temperature

Magnitude σ called Stefan's constant- Boltzmann:

σ = 5.6687·10 -8 J m - 2 s - 1 K – 4.

Energy emitted over time t absolutely black body with a radiating surface S at constant temperature T,

W=σT 4 St

If the body temperature changes over time, i.e. T = T(t), That

The Stefan-Boltzmann law indicates an extremely rapid increase in radiation power with increasing temperature. For example, when the temperature increases from 800 to 2400 K (i.e. from 527 to 2127 ° C), the radiation of a completely black body increases by 81 times. If a completely black body is surrounded by a medium with a temperature T 0, then the eye will absorb the energy emitted by the environment itself.

In this case, the difference between the power of emitted and absorbed radiation can be approximately expressed by the formula

U=σ(T 4 – T 0 4)

The Stefan-Boltzmann law is not applicable to real bodies, as observations show a more complex relationship R on temperature, as well as on the shape of the body and the condition of its surface.

2. Wien's law of displacement. Wavelength λ 0, which accounts for the maximum spectral density of black body radiation, is inversely proportional to the absolute temperature of the body:

λ 0 = or λ 0 T = b.

Constant b, called Wien's law constant, equal to b = 0.0028978 m K ( λ expressed in meters).

Thus, with increasing temperature, not only the total radiation increases, but, in addition, the distribution of energy across the spectrum changes. For example, at low body temperatures, mainly infrared rays are studied, and as the temperature increases, the radiation becomes reddish, orange and, finally, white. In Fig. Figure 2.1 shows the empirical distribution curves of the radiation energy of a black body over wavelengths at different temperatures: it is clear from them that the maximum spectral density of radiation shifts towards shorter waves with increasing temperature.

3. Planck's law. The Stefan-Boltzmann law and the Wien displacement law do not solve the main problem of how large the spectral radiation density is at each wavelength in the spectrum of a black body at temperature T. To do this, you need to establish a functional dependency And from λ And T.

Based on the idea of ​​the continuous nature of the emission of electromagnetic waves and on the law of uniform distribution of energy over degrees of freedom (accepted in classical physics), two formulas were obtained for the spectral density and radiation of a black body:

1) Wine formula

Where a And b- constant values;

2) Rayleigh-Jeans formula

u λT = 8πkT λ – 4 ,

Where k- Boltzmann constant. Experimental testing has shown that for a given temperature Wien's formula is correct for short waves (when λT very small and gives sharp convergences of experience in the region of long waves. The Rayleigh-Jeans formula turned out to be true for long waves and is completely inapplicable for short ones (Fig. 2.2).

Thus, classical physics was unable to explain the law of energy distribution in the radiation spectrum of an absolutely black body.

To determine the type of function u λТ completely new ideas about the mechanism of light emission were needed. In 1900, M. Planck hypothesized that absorption and emission of electromagnetic radiation energy by atoms and molecules is possible only in separate “portions”, which are called energy quanta. Magnitude of energy quantum ε proportional to the radiation frequency v(inversely proportional to wavelength λ ):

ε = hv = hc/λ

Proportionality factor h = 6.625·10 -34 J·s and is called Planck's constant. In the visible part of the spectrum for wavelength λ = 0.5 µm the value of the energy quantum is equal to:

ε = hc/λ= 3.79·10 -19 J·s = 2.4 eV

Based on this assumption, Planck obtained a formula for u λТ:

Where k– Boltzmann constant, With– speed of light in vacuum. l The curve corresponding to function (2.1) is also shown in Fig. 2.2.

From Planck's law (2.11) the Stefan-Boltzmann law and Wien's displacement law are obtained. Indeed, for the integral radiation density we obtain

Calculation using this formula gives a result that coincides with the empirical value of the Stefan-Boltzmann constant.

Wien's displacement law and its constant can be obtained from Planck's formula by finding the maximum of the function u λТ, why is the derivative of u λТ By λ , and is equal to zero. The calculation leads to the formula:

Calculation of constant b this formula also gives a result that coincides with the empirical value of the Wien constant.

Let us consider the most important applications of the laws of thermal radiation.

A. Thermal light sources. Most artificial light sources are thermal emitters (incandescent electric lamps, conventional arc lamps, etc.). However, these light sources are not very economical.

In § 1 it was said that the eye is sensitive only to a very narrow part of the spectrum (from 380 to 770 nm); all other waves do not have a visual sensation. The maximum sensitivity of the eye corresponds to the wavelength λ = 0.555 µm. Based on this property of the eye, one should require from light sources such a distribution of energy in the spectrum at which the maximum spectral radiation density would fall on the wavelength λ = 0.555 µm or so. If we take an absolutely black body as such a source, then using Wien’s displacement law we can calculate its absolute temperature:

Thus, the most advantageous thermal light source should have a temperature of 5200 K, which corresponds to the temperature of the solar surface. This coincidence is the result of the biological adaptation of human vision to the distribution of energy in the solar radiation spectrum. But even this light source efficiency(the ratio of the energy of visible radiation to the total energy of all radiation) will be small. Graphically in Fig. 2.3 this coefficient is expressed by the ratio of areas S 1 And S; square S 1 expresses the energy of radiation in the visible region of the spectrum, S- all radiation energy.

Calculations show that at a temperature of about 5000-6000 K, the light efficiency is only 14-15% (for an absolutely black body). At the temperature of existing artificial light sources (3000 K), this efficiency is only about 1-3%. Such a low “light output” of a thermal emitter is explained by the fact that during the chaotic movement of atoms and molecules, not only light (visible) waves are excited, but also other electromagnetic waves that do not have a light effect on the eye. Therefore, it is impossible to selectively force the body to emit only those waves to which the eye is sensitive: invisible waves are also emitted.

The most important of modern temperature light sources are incandescent electric lamps with tungsten filament. The melting point of tungsten is 3655 K. However, heating the filament to temperatures above 2500 K is dangerous, since tungsten at this temperature is very quickly atomized and the filament is destroyed. To reduce filament sputtering, it was proposed to fill the lamps with inert gases (argon, xenon, nitrogen) at a pressure of about 0.5 atm. This made it possible to raise the temperature of the filament to 3000-3200 K. At these temperatures, the maximum spectral density of radiation lies in the region of infrared waves (about 1.1 microns), therefore all modern incandescent lamps have an efficiency of slightly more than 1%.

B. Optical pyrometry. The laws of black body radiation outlined above make it possible to determine the temperature of this body if the wavelength is known λ 0 , corresponding to the maximum u λТ(according to Wien's law), or if the value of the integral radiation density is known (according to the Stefan-Boltzmann law). These methods of determining body temperature from its thermal radiation in the cabin optical pyrometry; they are especially useful when measuring very high temperatures. Since the mentioned laws apply only to an absolutely black body, optical pyrometry based on them gives good results only when measuring the temperatures of bodies that are close in their properties to an absolutely black body. In practice, these are factory furnaces, laboratory muffle furnaces, boiler furnaces, etc. Let's consider three ways to determine the temperature of thermal emitters:

A. Method based on Wien's displacement law. If we know the wavelength at which the maximum spectral density of radiation falls, then the body temperature can be calculated using formula (2.2).

In particular, the temperature on the surface of the Sun, stars, etc. is determined in this way.

For non-black bodies, this method does not give the true body temperature; if there is one maximum in the emission spectrum and we calculate T according to formula (2.2), then the calculation gives us the temperature of an absolutely black body, which has almost the same energy distribution in the spectrum as the body under test. In this case, the color of the radiation of an absolutely black body will be the same as the color of the radiation under study. This body temperature is called its color temperature.

The color temperature of an incandescent lamp filament is 2700-3000 K, which is very close to its true temperature.

b. Radiation method of measuring temperatures based on measuring the integral radiation density of the body R and calculating its temperature using the Stefan-Boltzmann law. The corresponding devices are called radiation pyrometers.

Naturally, if the radiating body is not absolutely black, then the radiation pyrometer will not give the true temperature of the body, but will show the temperature of an absolutely black body at which the integral radiation density of the latter is equal to the integral radiation density of the test body. This body temperature is called radiation, or energy, temperature.

Among the disadvantages of a radiation pyrometer, we point out the impossibility of using it to determine the temperatures of small objects, as well as the influence of the medium located between the object and the pyrometer, which absorbs part of the radiation.

V. I brightness method for determining temperatures. Its operating principle is based on a visual comparison of the brightness of the hot filament of the pyrometer lamp with the brightness of the image of the heated test body. The device is a telescope with an electric lamp placed inside, powered by a battery. Equality, visually observed through a monochromatic filter, is determined by the disappearance of the image of the thread against the background of the image of the hot body. The filament is regulated by a rheostat, and the temperature is determined by the ammeter scale, graduated directly to the temperature.

FEDERAL AGENCY FOR EDUCATION

state educational institution of higher professional education

"TYUMEN STATE OIL AND GAS UNIVERSITY"

Abstract on the discipline

"Technical Optics"

Topic: “Absolutely black body”

Completed by: student gr. OBDzs-07

Kobasnyan Stepan Sergeevich Checked by: teacher of the discipline

Sidorova Anastasia Eduardovna

Tyumen 2009

Absolutely black body- a physical abstraction used in thermodynamics, a body that absorbs all electromagnetic radiation incident on it in all ranges and does not reflect anything. Despite the name, a completely black body can itself emit electromagnetic radiation of any frequency and visually have color. The radiation spectrum of an absolutely black body is determined only by its temperature.

The blackest real substances, for example, soot, absorb up to 99% of incident radiation (i.e., have an albedo of 0.01) in the visible wavelength range, but they absorb infrared radiation much less well. Among the bodies of the Solar System, the Sun has the properties of an absolutely black body to the greatest extent. The term was introduced by Gustav Kirchhoff in 1862.

Black body model

Absolutely black bodies do not exist in nature, so in physics a model is used for experiments. It is a closed cavity with a small hole. Light entering through this hole will be completely absorbed after repeated reflections, and the hole will appear completely black from the outside. But when this cavity is heated, it will develop its own visible radiation.

Laws of black body radiation

Classic approach

The study of the laws of black body radiation was one of the prerequisites for the emergence of quantum mechanics.

Wien's first law of radiation

In 1893, Wilhelm Wien, based on the concepts of classical thermodynamics, derived the following formula:

Wien's first formula is valid for all frequencies. Any more specific formula (for example, Planck's law) must satisfy Wien's first formula.

From the first Wien formula one can derive the Wien displacement law (maximum law) and the Stefan-Boltzmann law, but one cannot find the values ​​of the constants included in these laws.

Historically, it was Wien’s first law that was called the displacement law, but currently the term “Wien’s displacement law” refers to the maximum law.

Wien's second law of radiation

In 1896, Wien derived the second law based on additional assumptions:

Experience shows that Wien's second formula is valid only in the limit of high frequencies (short wavelengths). It is a special case of Wien's first law.

Later, Max Planck showed that Wien's second law follows from Planck's law for high quantum energies, and also found the constants C 1 and C 2. Taking this into account, Wien's second law can be written as:

Rayleigh-Jeans law

An attempt to describe the radiation of a completely black body based on the classical principles of thermodynamics and electrodynamics leads to the Rayleigh-Jeans law:

This formula assumes a quadratic increase in the spectral density of radiation depending on its frequency. In practice, such a law would mean the impossibility of thermodynamic equilibrium between matter and radiation, since according to it all thermal energy would have to be converted into radiation energy in the short-wave region of the spectrum. This hypothetical phenomenon was called an ultraviolet catastrophe.

Nevertheless, the Rayleigh-Jeans radiation law is valid for the long-wave region of the spectrum and adequately describes the nature of the radiation. The fact of such correspondence can be explained only by using a quantum mechanical approach, according to which radiation occurs discretely. Based on quantum laws, we can obtain Planck’s formula, which will coincide with the Rayleigh-Jeans formula for

This fact is an excellent illustration of the principle of correspondence, according to which a new physical theory must explain everything that the old one was able to explain.

Planck's law

Dependence of black body radiation power on wavelength

The radiation intensity of an absolutely black body, depending on temperature and frequency, is determined by Planck's law :

Where I (ν) dν - radiation power per unit area of ​​the radiating surface in the frequency range from ν to ν + d ν.

Equivalently,

Where u (λ) dλ - radiation power per unit area of ​​the emitting surface in the wavelength range from λ to λ + d λ.

Stefan-Boltzmann law

The total energy of thermal radiation is determined Stefan-Boltzmann law :

Where j is the power per unit area of ​​the radiating surface, and

Thus, an absolutely black body at T= 100 K emits 5.67 watts per square meter of its surface. At a temperature of 1000 K, the radiation power increases to 56.7 kilowatts per square meter.

Wien's displacement law

The wavelength at which the radiation energy of an absolutely black body is maximum is determined by Wien's displacement law :

Where T is the temperature in Kelvin, and λ max is the wavelength with maximum intensity in meters.

So, if we assume as a first approximation that human skin is close in properties to an absolutely black body, then the maximum of the radiation spectrum at a temperature of 36°C (309 K) lies at a wavelength of 9400 nm (in the infrared region of the spectrum).

The apparent color of completely black bodies at different temperatures is shown in the diagram.

Blackbody radiation

Electromagnetic radiation that is in thermodynamic equilibrium with a blackbody at a given temperature (for example, radiation inside a cavity in a blackbody) is called blackbody (or thermal equilibrium) radiation. Equilibrium thermal radiation is homogeneous, isotropic and non-polarized, there is no energy transfer in it, all its characteristics depend only on the temperature of the absolutely blackbody emitter (and, since blackbody radiation is in thermal equilibrium with this body, this temperature can be attributed to radiation). The volumetric energy density of blackbody radiation is equal to

Blackbody chromaticity

Note: The colors are given in comparison with diffuse daylight (D 65). The actual perceived color may be distorted by the eye's adaptation to lighting conditions.

To record the laws of radiation and absorption, the concept of an absolutely black body is introduced. Absolutely black body called an imaginary body that absorbs all the energy incident on it at any temperature. The emissive and absorptive abilities are interconnected, this connection is expressed Kirchhoff's law .

For all bodies at a given temperature, the ratio of emissive ability to absorptive ability is a constant value and equal to the emissive ability of an absolutely black body.

R depends on temperature, this dependence is expressed Stefan-Boltzmann law:

Stefan-Boltzmann constant

The dependence of the radiation wavelength on body temperature is expressed Wien's law .

The wavelength corresponding to the maximum radiation of a completely black body is inversely proportional to its thermodynamic temperature.

b= , constant Guilt

Questions for self-control

1. What is called the photoelectric effect?

2. Formulate the laws of the external photoelectric effect.

3. What is called luminescence?

4. What is Stokes' rule?

5. Write down the mass and momentum of the photon.

6. What is called a perfect black body?

7. Formulate the laws of black body radiation.

8. What is the duality of the nature of light?

9. Formulate the basic laws of geometric optics.

10. What is the phenomenon of diffraction?

11. What is the phenomenon of interference?

12. What kind of light is called polarized?

13. What is called light dispersion?

14. Name the main photometric characteristics.