>> Light pressure

§ 91 PRESSURE LIGHT

Maxwell, based on the electromagnetic theory of light, predicted that light should exert pressure on obstacles.

Under the influence electric field wave incident on the surface of a body, such as a metal, a free electron moves in the direction opposite to the vector (Fig. 11.7). A moving electron is affected by the Lorentz force directed in the direction of wave propagation. Total Strength, acting on the electrons of the metal surface, and determines the force of light pressure.

To prove the validity of Maxwell's theory, it was important to measure the pressure of light. Many scientists have tried to do this, but without success, because the light pressure is very low. On a bright sunny day, a force equal to only 4 10 -6 N acts on a surface with an area of ​​\u200b\u200b1m 2. The pressure of light was first measured by the Russian physicist Pyotr Nikolaevich Lebedev in 1900.

Lebedev Petr Nikolaevich (1866-1912)- Russian physicist who first measured the pressure of light on solids and gases. These works quantitatively confirmed Maxwell's theory. In an effort to find new experimental evidence for the electromagnetic theory of light, he received electromagnetic waves of millimeter wavelengths and investigated all their properties. Created the first in Russia physical school. Many outstanding Soviet scientists were his students. The Physics Institute of the USSR Academy of Sciences (FIAN) bears Lebedev's name.

Lebedev's device consisted of a very light rod on a thin glass thread, but light wings were glued to the edges (Fig. 11.8). The entire device was placed in a vessel, from where the air was pumped out. The light fell on the wings located on one side of the rod. The value of pressure could be judged by the angle of twist of the thread. Difficulties in accurately measuring the pressure of light were associated with the impossibility of pumping out all the air from the vessel (the movement of air molecules, caused by unequal heating of the wings and walls of the vessel, leads to additional torques). In addition, the twisting of the thread is affected by the uneven heating of the sides of the wings (the side facing the light source heats up more than opposite side). Molecules reflecting off the hotter side impart more momentum to the winglet than molecules reflecting off the cooler side.

Lebedev managed to overcome all these difficulties, despite low level the then experimental technique, taking a very large vessel and very thin wings. Eventually the existence of light pressure on solids was proven and measured. The obtained value coincided with the one predicted by Maxwell. Subsequently, after three years of work, Lebedev managed to carry out an even more subtle experiment: to measure the pressure of light on gases.

The advent of the quantum theory of light made it possible to more simply explain the cause of light pressure. Photons, like particles of matter that have a rest mass, have momentum. When absorbed by their body, they transfer their impulse to it. According to the law of conservation of momentum, the momentum of the body becomes equal to the momentum of the absorbed photons. Therefore, a body at rest begins to move. A change in the momentum of a body means, according to Newton's second law, that a force acts on the body.

Lebedev's experiments can be regarded as experimental evidence that photons have momentum.

Although light pressure is very small under normal conditions, its effect can nevertheless be significant. Inside stars at a temperature of several tens of millions of Kelvin, the pressure of electromagnetic radiation must reach enormous values. Light pressure forces, along with gravitational forces, play a significant role in interstellar processes.

According to Maxwell's electrodynamics, the pressure of light arises due to the action of the Lorentz force on the electrons of the medium, which oscillate under the action of an electric field electromagnetic wave. From the point of view of quantum theory, pressure appears as a result of the transfer of photon impulses to the body during their absorption.

Myakishev G. Ya., Physics. Grade 11: textbook. for general education institutions: basic and profile. levels / G. Ya. Myakishev, B. V. Bukhovtsev, V. M. Charugin; ed. V. I. Nikolaev, N. A. Parfenteva. - 17th ed., revised. and additional - M.: Education, 2008. - 399 p.: ill.

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This video tutorial is dedicated to the topic “The pressure of light. Lebedev's experiments. Lebedev's experiments made a huge impression on the scientific world, because thanks to them, the pressure of light was measured for the first time and the validity of Maxwell's theory was proved. How did he do it? The answer to this and many others interesting questions related to the quantum theory of light, you can learn from this fascinating physics lesson.

Theme: Light pressure

Lesson: Light pressure. Lebedev's experiments

For the first time, the hypothesis of the existence of light pressure was put forward by Johannes Kepler in the 17th century to explain the phenomenon of comet tails when they fly near the Sun.

Maxwell, based on the electromagnetic theory of light, predicted that light should exert pressure on an obstacle.

Under the action of the electric field of the wave, the electrons in the bodies oscillate - it is formed electricity. This current is directed along the electric field strength. Orderly moving electrons are affected by the Lorentz force from the side magnetic field, directed in the direction of wave propagation - this is light pressure force(Fig. 1).

Rice. 1. Maxwell's experiment

To prove Maxwell's theory, it was necessary to measure the pressure of light. For the first time, light pressure was measured by the Russian physicist Pyotr Nikolaevich Lebedev in 1900 (Fig. 2).

Rice. 2. Petr Nikolaevich Lebedev

Rice. 3. Lebedev device

Lebedev's device (Fig. 3) consists of a light rod on a thin glass thread, along the edges of which light wings are attached. The whole device was placed in a glass vessel, from which the air was pumped out. Light falls on the wings located on one side of the rod. The value of pressure can be judged by the angle of twist of the thread. The difficulty of accurately measuring the pressure of light was due to the fact that it was impossible to pump out all the air from the vessel. During the experiment, the movement of air molecules began, caused by unequal heating of the wings and walls of the vessel. Wings cannot be hung absolutely vertically. Heated air flows rise up, act on the wings, which leads to additional torques. Also, the twisting of the thread is affected by the non-uniform heating of the sides of the wings. The side facing the light source heats up more than the opposite side. Molecules bouncing off the hotter side impart more momentum to the winglet.

Rice. 4. Lebedev device

Rice. 5. Lebedev device

Lebedev managed to overcome all difficulties, despite the low level of experimental technology at that time. He took a very large vessel and very thin wings. The wing consisted of two pairs of thin platinum circles. One of the circles of each pair was shiny on both sides. The other sides had one side coated with platinum black. At the same time, both pairs of circles differed in thickness.

To exclude convection currents, Lebedev directed beams of light to the wings from one side or the other. Thus, the forces acting on the wings were balanced (Fig. 4-5).

Rice. 6. Lebedev device

Rice. 7. Lebedev device

Thus, the pressure of light on solid bodies was proved and measured (Fig. 6-7). The value of this pressure coincided with Maxwell's predicted pressure.

Three years later, Lebedev managed to make another experiment - to measure the pressure of light on gases (Fig. 8).

Rice. 8. Installation for measuring the pressure of light on gases

Lord Kelvin: “Perhaps you know that all my life I fought with Maxwell, not recognizing his light pressure, and now your Lebedev forced me to surrender before his experiments.”

The advent of the quantum theory of light made it possible to more simply explain the cause of light pressure.

Photons have momentum. When absorbed by their body, they transfer their impulse to it. Such an interaction can be regarded as an absolutely inelastic impact.

A force acts on the surface from each photon:

Light pressure on the surface:

Interaction of a photon with a mirror surface

In the case of this interaction, an absolutely elastic interaction is obtained. When a photon falls on a surface, it is reflected from it with the same speed and momentum with which it fell on this surface. The momentum change will be twice as large as when a photon falls on a black surface, the light pressure will double.

In nature, there are no substances whose surface would completely absorb or reflect photons. Therefore, to calculate the pressure of light on real bodies, it is necessary to take into account that some of the photons will be absorbed by this body, and some will be reflected.

Lebedev's experiments can be regarded as experimental evidence that photons have momentum. Although under normal conditions the light pressure is very small, its effect can be significant. Based on the pressure of the Sun, a sail was developed for spaceships, which will allow you to move in space under the pressure of light (Fig. 11).

Rice. 11. Spaceship sail

The pressure of light, according to Maxwell's theory, arises as a result of the action of the Lorentz force on electrons that make oscillatory motions under the influence of the electric field of an electromagnetic wave.

From the point of view of quantum theory, the pressure of light arises as a result of the interaction of photons with the surface on which they fall.

The calculations that were carried out by Maxwell coincided with the results that Lebedev produced. This vividly proves the quantum-wave dualism of light.

Crookes' experiments

Lebedev first discovered the pressure of light experimentally and was able to measure it. The experiment was incredibly difficult, but there is a scientific toy - the Crookes experiment (Fig. 12).

Rice. 12. Crookes experiment

A small propeller, consisting of four petals, is located on the needle, which is covered with a glass cap. If this propeller is illuminated with light, it starts to rotate. If you look at this propeller in the open air, when the wind blows on it, its rotation would not surprise anyone, but in this case, the glass dome does not allow air currents to act on the propeller. Therefore, the cause of its movement is light.

English physicist William Crookes accidentally created the first light spinner.

In 1873, Crookes decided to determine the atomic weight of the element Thallium and weigh it by a very accurate scales. To prevent random air currents from distorting the weighing pictures, Crookes decided to hang the rocker arms in a vacuum. I did it and was amazed, since its thinnest scales were sensitive to heat. If the heat source was under the object, it reduced its weight, if above, it increased it.

Having improved this accidental experience of his, Crookes came up with a toy - a radiometer (light mill). The Crookes radiometer is a four-bladed impeller balanced on a needle inside a glass bulb with a slight vacuum. When a light beam hits the blade, the impeller starts to rotate, which is sometimes incorrectly explained by light pressure. In fact, the cause of torsion is the radiometric effect. The emergence of a repulsive force due to the difference in kinetic energies of gas molecules incident on the consecrated (heated) side of the blade and on the opposite unlit (colder) side.

  1. The pressure of light and the pressure of circumstances ().
  2. Pyotr Nikolaevich Lebedev ().
  3. Crookes radiometer ().

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The quantum theory of light explains light pressure as the result of the transfer of momentum by photons to atoms or molecules of matter.

Let on the surface of the square S normal to her every second falls

N photon frequency v . Every photon has momentum hv/c . If

R is the reflection coefficient of the surface, then pN photons will be reflected from the surface, ( 1-p) N photons are absorbed.

Each absorbed quantum of light will impart momentum to the surface hv/c , and each reflected is an impulse [(hv/c) - (-hv/c)] = 2hv/c , since when reflected, the direction of the photon momentum changes to the opposite and the momentum transmitted by it to the particles of matter is 2hv/s . Full the momentum received by the surface of the body will be

Let's calculate the light pressure. To do this, (20.18) we divide the "wing" on the area S: (20.19)

If we take into account that hvN/S = Ee, then formula (20.19) takes the form

(20.20)

Expressions (20.17) and (20.20), derived in terms of electromagnetic and quantum theories, match up.

Experimentally, the validity of these results was proved by the experiments of P.N. Lebedev.

The pressure of natural light is very low. If the absorption coefficient of the surface is close to unity, then the pressure exerted by the sun's rays on such surfaces located on the Earth is approximately

5 10 Pa (i.e. 3.7 10 mmHg) . This pressure is ten orders of magnitude less atmospheric pressure at the surface of the earth.

P. N. Lebedev was able to measure such a low pressure only by demonstrating exceptional ingenuity and skill in setting up and conducting an experiment.

Light pressure plays no role in the phenomena that we encounter in life. But in cosmic and microscopic systems its role is essential.

In the microcosm, light pressure manifests itself in the light output experienced by an excited atom when it emits light. The gravitational attraction of the outer layers of stellar matter to its center is balanced by a force, a significant contribution to which is made by the pressure of light coming from the depths of the star outward.

The chemical action of light

As a result of the action of light, chemical transformations occur in some substances. - photochemical reactions . Photochemical transformations are very diverse. Under the action of light, complex molecules can decompose into their constituent parts (for example, silver bromide - into silver and bromine) or. on the contrary, complex molecules are formed (for example, if a mixture of chlorine and hydrogen is illuminated, the reaction of the formation of hydrogen chloride proceeds so violently that it is accompanied by an explosion).

Many of the photochemical reactions play an important role in nature and technology. The main one is photochemical decomposition of carbon dioxide that occurs under the influence of light in the green parts of plants. This reaction has great value, because it provides the carbon cycle, without which the long existence of organic life on Earth is impossible. As a result of the vital activity of animals and plants (respiration), there is a continuous process of carbon oxidation (formation CO2 ). The reverse process of carbon reduction occurs under the influence of light in the green parts of plants. This reaction proceeds according to the scheme 2CO2 2CO + O2

The photochemical reaction of the decomposition of silver bromide underlies photography and all its scientific and technical applications, the phenomenon of fading of paints, which is mainly reduced to the photochemical oxidation of these paints, has a very great importance to understand the processes occurring in the human and animal eye and underlying visual perception. Very many photochemical reactions are now used in chemical production and thus acquire direct industrial significance.

Today we will devote a conversation to such a phenomenon as the pressure of light. Consider the prerequisites for discovery and consequences for science.

light and color

The mystery of human abilities has excited people since ancient times. How does the eye see? Why do colors exist? What is the reason that the world is the way we perceive it? How far can a person see? Experiments with the decomposition of a solar ray into a spectrum were carried out by Newton in the 17th century. He also laid a strict mathematical foundation in a number of disparate facts that at that time were known about light. And Newtonian theory predicted a lot: for example, discoveries that only quantum physics explained (the deflection of light in a gravitational field). But the physics of that time did not know and did not understand the exact nature of light.

wave or particle

Since scientists all over the world began to penetrate into the essence of light, there has been a debate: what is radiation, a wave or a particle (corpuscle)? Some facts (refraction, reflection and polarization) confirmed the first theory. Others (rectilinear propagation in the absence of obstacles, light pressure) - the second. However, only quantum physics was able to calm this dispute by combining the two versions into one common one. states that any microparticle, including a photon, has both the properties of a wave and a particle. That is, a quantum of light has such characteristics as frequency, amplitude and wavelength, as well as momentum and mass. Let's make a reservation right away: photons have no rest mass. Being a quantum of the electromagnetic field, they carry energy and mass only in the process of movement. This is the essence of the concept of "light". Physics today has explained it in sufficient detail.

Wavelength and Energy

The concept of "wave energy" was mentioned a little higher. Einstein convincingly proved that energy and mass are identical concepts. If a photon carries energy, it must have mass. However, a quantum of light is a “cunning” particle: when a photon collides with an obstacle, it completely gives up its energy to matter, becomes it and loses its individual essence. At the same time, certain circumstances (strong heating, for example) can cause the previously dark and calm interiors of metals and gases to emit light. The momentum of a photon, a direct consequence of the presence of mass, can be determined using the pressure of light. researcher from Russia, convincingly proved this amazing fact.

Lebedev's experience

The Russian scientist Petr Nikolaevich Lebedev in 1899 made the following experiment. On a thin silver thread he hung a crossbar. To the ends of the crossbar, the scientist attached two plates of the same substance. These were silver foil, and gold, and even mica. Thus, a kind of scales were created. Only they measured the weight not of the load that presses from above, but of the load that presses from the side on each of the plates. Lebedev placed this entire structure under a glass cover so that the wind and random fluctuations in air density could not affect it. Further, I would like to write that he created a vacuum under the lid. But at that time, even an average vacuum was impossible to achieve. So we say that he created under the glass cover strongly And alternately illuminated one plate, leaving the other in shadow. The amount of light directed at the surfaces was predetermined. From the deflection angle, Lebedev determined what impulse transmitted the light to the plates.

Formulas for determining the pressure of electromagnetic radiation at normal beam incidence

To begin with, what is a "normal fall"? Light is incident on a surface normally if it is directed strictly perpendicular to the surface. This imposes restrictions on the problem: the surface must be perfectly smooth, and the radiation beam must be directed very accurately. In this case, the pressure is calculated:

k is the transmittance, ρ is the reflection coefficient, I is the intensity of the incident light beam, c is the speed of light in vacuum.

But, probably, the reader has already guessed that such an ideal combination of factors does not exist. Even if we do not take into account the ideality of the surface, it is rather difficult to organize the incidence of light strictly perpendicularly.

Formulas for determining the pressure of electromagnetic radiation when it falls at an angle

The pressure of light on a mirror surface at an angle is calculated using a different formula, which already contains elements of vectors:

p= ω ((1-k)i+ρi') cos ϴ

The values ​​p, i, i' are vectors. In this case, k and ρ, as in the previous formula, are the transmission and reflection coefficients, respectively. The new values ​​mean the following:

  • ω - volume density of radiation energy;
  • i and i' - unit vectors, which show the direction of the incident and reflected beam of light (they set the directions in which the acting forces should be added);
  • ϴ is the angle to the normal at which the light ray falls (and, accordingly, is reflected, since the surface is mirrored).

We remind the reader that the normal is perpendicular to the surface, so if the problem is given the angle of incidence of light to the surface, then ϴ is 90 degrees minus the given value.

Application of electromagnetic radiation pressure phenomenon

A schoolboy who studies physics finds many formulas, concepts and phenomena boring. Because, as a rule, the teacher tells the theoretical aspects, but rarely can give examples of the benefits of certain phenomena. Let's not blame the school mentors for this: they are severely limited by the program, during the lesson it is necessary to tell extensive material and still have time to check the knowledge of the students.

Nevertheless, the object of our study has many interesting applications:

  1. Now almost every student in the laboratory of his educational institution can repeat Lebedev's experiment. But then the coincidence of experimental data with theoretical calculations was a real breakthrough. The experiment, made for the first time with a 20% error, allowed scientists around the world to develop a new branch of physics - quantum optics.
  2. Obtaining high-energy protons (for example, for irradiation different substances) by accelerating thin films with a laser pulse.
  3. Pressure accounting electromagnetic radiation Sun on the surface of near-Earth objects, including satellites and space stations, allows you to correct their orbit with greater accuracy and prevents these devices from falling to Earth.

The above applications exist now in real world. But there are also potential opportunities that have not yet been realized, because the technology of mankind has not yet reached the required level. Among them:

  1. With its help, it would be possible to move quite large loads in near-Earth and even near-solar space. Light gives a small impulse, but with the right position of the surface of the sail, the acceleration would be constant. In the absence of friction, it is enough to gain speed and deliver goods to the desired point in the solar system.
  2. Photonic engine. This technology, perhaps, will allow a person to overcome the attraction of his own star and fly to other worlds. The difference from a solar sail is that an artificially created device, for example, a thermonuclear engine, will generate solar impulses.