With what force does light press on the objects it illuminates? Why don't we feel this power? How can it be applied? What causes light pressure? In this article you will find answers to these questions.
Scientists use two different models to describe light in different cases. When propagating, light is represented as an electromagnetic wave, and when interacting with objects - as small particles (corpuscles). These particles were called photons. Various interpretations of the light have been called wave-particle duality. This means that if we want to describe how light propagates in space (for example, Young's experiment with the interference of light), then we consider light electromagnetic wave. But, if we want to describe the interaction of light with matter (for example, the external photoelectric effect), then we consider light a stream of corpuscles, or rather photons.
Now imagine this situation: a ping-pong ball is first thrown at concrete wall, then into a wall coated with a very sticky substance. In the first case, the ball will bounce off the wall at almost the same speed as it hit it, and in the second case, it will stick to the wall. In which of these cases will the wall “push away” the ball with greater force? Naturally, when the wall is concrete. Indeed, in this case, she must not only stop the ball, but also “launch” it back. Because the action force is equal to the reaction force, then the ball will have a stronger effect on the concrete wall than on the sticky one.
Now let's do a little mental experiment. Let's imagine a crossbar that can rotate around a vertical axis in a horizontal plane (see figure).
We will hang two round plates on its sides. One is concrete, the second is sticky. Let's throw a ball into these plates at the same time. Since the impact forces will be different for the plates, the crossbar will begin to rotate around the vertical axis. Based on its rotation speed and the material of the plates, it will be possible to judge the magnitude of the impact forces. The great scientist P.N. did the same. Lebedev in his real experiment. Only instead of balls he used light (when interacting with matter, light is described as a stream of photons), instead of a crossbar - a very light silver rocker, instead of a concrete plate - a very light mirror wing, and instead of a sticky plate - a very light matte wing. The mirrored wing pushed the light back, while the matte wing simply stopped it. Knowing the reflection coefficients for the two surfaces and the rotation speed of the rocker, Lebedev estimated the light pressure. Light pressure can be calculated using the formula:
Where J – light intensity, r – light reflectance, With – speed of light in vacuum. For mirror surfaces r = 1, with complete absorption (for an absolutely black body) r = 0.
It is impossible to notice the light pressure under normal conditions because it is too low. Thus, the force with which the light of the Sun acts on the entire illuminated surface of our planet is ten thousand billion times less than the force with which the Sun attracts the Earth. Light presses down on our planet with a gravity force of 60,000 tons. The pressure of light on the Earth (force per 1 square meter in the SI system) is one millionth of a Pascal. For comparison, atmospheric pressure = 101000 Pascal.
You say: “Why then measure such small quantities at all? How can light pressure be applied? From a theoretical point of view, the fact of the existence of light pressure is proof of the truth of certain statements electromagnetic theory light, and also confirms the existence of a relativistic mass of light. But the use of light pressure can be found in space travel. It turns out that if spaceship with a solar sail will begin its journey from our planet, then in a few months it will reach Mars and Jupiter. And all this is only due to the Sun. This will require a solar sail, square meter area of which should weigh less than 1 gram. His total area should be about 1 km square. The sail will then have an acceleration of 1 mm per second squared. It is small, but in six months the speed of the sail will reach the speed of Voyager 1, which is 17 km/sec.
Thus, the only problem with using solar sails as free space transport is its big sizes and small mass. New materials are needed that can solve this problem. If instead of the light of the Sun we use the light of artificial high-power lasers, we can achieve even higher sail speeds at large values its mass. That is why solar sails are considered by scientists as the best remedy for travel in space.
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Light is not only absorbed and reflected by the substance, but also creates pressure on the surface of the body. Back in 1604, the German astronomer J. Kepler explained the shape of the comet's tail by the action of light pressure (Fig. 1). The English physicist J. Maxwell, 250 years later, calculated the light pressure on bodies using the theory he developed electro magnetic field. According to Maxwell's calculations, it turned out that if light energy $E,$ falls per $1$ perpendicular to a unit area with reflection coefficient $R$, then the light exerts pressure $p,$ expressed by the dependence: $p=\frac(E)(c)( 1+R)$ N/m 2 - speed of light. This formula can also be obtained by considering light as a stream of photons interacting with a surface (Fig. 2).
Some scientists doubted Maxwell's theoretical calculations, and for a long time it was not possible to verify his result experimentally. In mid-latitudes at solar noon, on a surface that fully reflects light rays, a pressure equal to only $4.7⋅10^(−6)$ N/m 2 is created. For the first time, light pressure was measured in 1899 by the Russian physicist P. N. Lebedev. He hung two pairs of wings on a thin thread: the surface of one of them was blackened, and the other was mirrored (Fig. 3). The light was almost completely reflected from the mirror surface, and its pressure on the mirror wing was twice as large ($R=1$) than on the blackened one ($R=0$). A moment of force was created that rotated the device. By the angle of rotation one could judge the force acting on the wings, and therefore measure the light pressure.
The experiment is complicated by extraneous forces arising when the device is illuminated, which are thousands of times greater than the light pressure unless special precautions are taken. One of these forces is associated with the radiometric effect. This effect occurs due to the temperature difference between the illuminated and dark sides of the wing. The light-heated side reflects the residual gas molecules at a faster rate than the cooler, unlit side. Therefore, the gas molecules transfer a greater impulse to the illuminated side and the wings tend to turn in the same direction as under the influence of light pressure - a false effect occurs. P. N. Lebedev reduced the radiometric effect to a minimum by making wings from thin foil that conducts heat well and placing them in a vacuum. As a result, both the difference in momentum transmitted by individual molecules of black and shiny surfaces (due to a smaller temperature difference between them) and the total number of molecules falling on the surface (due to low gas pressure) decreased.
Lebedev's experimental studies supported Kepler's assumption about the nature of cometary tails. As the radius of a particle decreases, its attraction to the Sun decreases in proportion to the cube, and the light pressure decreases in proportion to the square of the radius. Small particles will experience repulsion from the Sun regardless of the distance $r$ from it, since the radiation density and gravitational attractive forces decrease according to the same law $1/r^2.$ Light pressure limits the maximum size of stars existing in the Universe. As the mass of a star increases, the gravity of its layers toward the center increases. Therefore, the inner layers of stars are greatly compressed, and their temperature increases to millions of degrees. Naturally, this significantly increases the outward light pressure inner layers. U normal stars there is a balance between gravitational forces, stabilizing the star, and the forces of light pressure tending to destroy it. For stars of very large masses, such equilibrium does not occur; they are unstable, and they should not exist in the Universe. Astronomical observations have confirmed: the “heaviest” stars have exactly the maximum mass that is still allowed by the theory, which takes into account the balance of gravitational and light pressure inside the stars.
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The physical phenomenon - the pressure of light on a surface - can be considered from two positions - the corpuscular and wave theories of light. According to the corpuscular (quantum) theory of light, a photon is a particle and has momentum, which, when the photon hits a surface, is fully or partially transferred to the surface. According to the wave theory, light is an electromagnetic wave, which, when passing through a material, has an effect on charged particles (Lorentz force), which explains the pressure of light in this theory.
Light of wavelength 620 nm is incident normally on a blackened surface and exerts a pressure of 0.1 μPa. How many photons fall on a surface with an area of 5 cm 2 in a time of 10 s?
Light falls normally on a mirror surface and exerts a pressure of 40 μPa on it. What is the irradiance of the surface?
Light of wavelength 600 nm is incident normally on a mirror surface and exerts a pressure of 4 μPa. How many photons hit a surface with an area of 1 mm 2 in a time of 10 s?
Light with a wavelength of 590 nm is incident on a mirror surface at an angle of 60 degrees. Luminous flux density 1 kW/m2. Determine the light pressure on the surface.
The source is located at a distance of 10 cm from the surface. The light pressure on the surface is 1 mPa. Find the power of the source.
A luminous flux of 0.8 W falls normally onto a mirror surface with an area of 6 cm2. Find the pressure and force of light pressure.
A luminous flux of 0.9 W falls normally on a mirror surface. Find the force of light pressure on this surface.
Light falls normally on a surface with a reflectance of 0.8. The light pressure exerted on this surface is 5.4 μPa. What energy will be brought by photons incident on a surface with an area of 1 m2 in a time of 1 s?
Find the light pressure exerted on the blackened surface of the incandescent lamp bulb from the inside. Consider the flask to be a sphere with a radius of 10 cm, and the lamp spiral to be a point light source with a power of 1 kW.
A luminous flux of 120 W/m2 falls normally on the surface and exerts a pressure of 0.5 μPa. Find the surface reflectance.
Light falls normally onto a perfectly reflective surface of area 5 cm2. In a time of 3 minutes, the energy of the incident light is 9 J. Find the light pressure.
Light falls on a mirror surface with an area of 4.5 cm2. Energy illumination of the surface 20 W/cm2. What impulse will the photons transmit to the surface in 5 s?
Light falls normally on a blackened surface and brings energy of 20 J in 10 minutes. The surface area is 3 cm2. Find the surface irradiance and light pressure.
Light with a flux power of 0.1 W/cm2 falls on a mirror surface at an incidence angle of 30 degrees. Determine the light pressure on the surface.
When electromagnetic waves fall on a surface, they exert pressure on that surface. The pressure of light can be explained both from an electromagnetic point of view and within the framework of quantum theory.
Let a normally plane electromagnetic wave fall on the metal surface, then the electric and magnetic field vectors of such a wave are parallel to the surface. Under the influence electric field E electrons begin to move parallel to the surface. Moreover, for each electron moving at a speed , from the side of the magnetic field of a light wave with induction Lorentz force acts
directed into the metal perpendicular to its surface. Thus, the light wave must produce pressure on the surface of the metal.
Within the framework of quantum photon theory, light pressure is due to the fact that each photon not only carries energy, but also has momentum . Each absorbed photon transfers its momentum to the surface
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and each reflected impulse is doubled
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Let a flux of photons fall normally onto the surface of some body N f (N f- the number of photons incident on a unit area per unit time). If the surface of a body has a reflectance coefficient, then per unit time photons will be reflected from it, and photons will be absorbed by the surface. The impulse received by a unit surface area of a body per unit time is equal to
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According to Newton's second law, there is a force normal to the surface (V in this case is the pressure force), and the magnitude - pressure. Thus, the light pressure is equal to
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Magnitude, equal to the product photon energy ħw per number of photons N f incident per unit area of the body per unit time is the flux density of light energy R. The same value can be obtained by multiplying average density energy
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We have already discussed this formula for and earlier when we considered the pressure of electromagnetic waves.
Example. Let's determine the pressure R sunlight onto a blackened plate located perpendicular to the sun's rays and located outside earth's atmosphere near the Earth.
Solar constant, that is, solar energy flux density electromagnetic radiation near the Earth outside its atmosphere, approximately equal 
. The blackened plate absorbs almost everything, that is, for evaluation, you can put . Hence the pressure
Light pressure plays a huge role in the orientation of comet tails relative to the Sun. Dust particles and gas molecules present in comets experience light pressure from sun rays, as a result of which peculiar forms of cometary tails are formed, oriented in the opposite direction from the Sun. (It is currently assumed that the phenomenon of comet tail formation is partly determined by the "proton" wind emanating from the Sun.)
Rice. 2.20. Light pressure deflects the comet's tail away from the Sun
Rice. 2.21. Project of a solar sail in Earth orbit, driven by light pressure
Thus, both electromagnetic (wave) and photon (quantum) theories solve the question of the mechanism and laws of light pressure with equal success.
Let's summarize:
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1. In the phenomena of light propagation and reflection (diffraction and interference), light behaves like a wave with typical wave characteristics such as frequency and wavelength . 2. In the phenomena of emission and transfer of energy, light behaves like a particle characterized by energy and momentum . 3. Planck's constant numerically connects corpuscular characteristics with wave characteristics. |
Therefore, we have to recognize the dual nature of the photon. So far in our course this unusual property - wave-particle duality - set for light only.
The hypothesis about the existence of light pressure was first put forward by I. Kepler in the 17th century to explain the behavior of comet tails when they fly near the Sun. In 1873, Maxwell gave a theory of the pressure of light within the framework of his classical electrodynamics. Light pressure was first studied experimentally by P. N. Lebedev in 1899. In his experiments, a rotary scale was suspended from a thin silver thread in an evacuated vessel, to the rocker arms of which thin disks made of mica and various metals were attached. The main difficulty was to isolate light pressure against the background of radiometric and convective forces (forces caused by the difference in temperature of the surrounding gas on the illuminated and unlit sides). By alternating irradiation different sides Lebedev leveled the radiometric forces and obtained a satisfactory (±20%) agreement with Maxwell’s theory. Later, in 1907-1910. Lebedev conducted more accurate experiments to study the pressure of light in gases and also obtained acceptable agreement with the theory.
According to today's concepts, light has wave-particle duality, that is, it exhibits the properties of particles (photons) and the properties of waves (electromagnetic radiation).
If we consider light as a stream of photons, then, according to the principles of classical mechanics, particles, when hitting a body, must transfer momentum to it, in other words, exert pressure. This pressure is sometimes called radiation pressure.
To calculate light pressure, you can use the following formula:
where is the amount of radiant energy falling normally onto 1 m² of surface in 1 s; - speed of light, - reflection coefficient.
If light falls at an angle to the normal, then the pressure can be expressed by the formula:
where is the volumetric radiation energy density, is the reflection coefficient, is the unit vector of the direction of the incident beam, is the unit vector of the direction of the reflected beam.
For example, the tangential component of the light pressure force on a unit area will be equal to:
The normal component of the light pressure force on a unit area will be equal to:
The ratio of the normal and tangential components is equal to:
Possible applications include solar sail and gas separation.
Light pressure- Light pressure. Scheme for separating gases using resonant light pressure (the frequency of laser light is equal to the frequency of the atomic transition). Resonant atoms under the influence of light, having received a directed impulse from light quanta, will go into the far... ... Illustrated encyclopedic Dictionary
Light pressure- the pressure exerted by light on reflecting or absorbing bodies. D. s. was first experimentally discovered and measured by P. N. Lebedev (1899). The value of D. s. even for the strongest light sources (Sun, electric arc) insignificant... ... Great Soviet Encyclopedia
LIGHT PRESSURE- Pressure exerted by light on bodies that reflect or absorb light. Light pressure is the result of the transfer of momentum to a body by photons absorbed or reflected by it. When solar radiation acts on macroscopic bodies, it is extremely small... ... Big Encyclopedic Dictionary
LIGHT PRESSURE- (see LIGHT PRESSURE). Physical encyclopedic dictionary. M.: Soviet encyclopedia. Editor-in-chief A. M. Prokhorov. 1983 ... Physical encyclopedia
light pressure- the pressure exerted by light on bodies that reflect or absorb light, particles, as well as individual molecules and atoms. The hypothesis about the pressure of light was first put forward (1619) by I. Kepler to explain the deflection of the tails of comets flying near the Sun.... ... encyclopedic Dictionary
light pressure- šviesos slėgis statusas T sritis Standartizacija ir metrologija apibrėžtis Slėgis, kurį kuria šviesa veikdama tam tikrą paviršių. atitikmenys: engl. light pressure vok. Lichtdruck, m rus. light pressure, n; light pressure, n pranc. pressure de… Penkiakalbis aiškinamasis metrologijos terminų žodynas
light pressure- šviesos slėgis statusas T sritis fizika atitikmenys: engl. light pressure vok. Lichtdruck, m rus. light pressure, n; light pressure, n pranc. pression de la lumière, f … Fizikos terminų žodynas
LIGHT PRESSURE- the pressure produced by light on bodies that reflect or absorb light, particles, as well as parts. molecules and atoms. Hypothesis about D. s. was first expressed (1619) by I. Kepler to explain the deflection of the tails of comets flying near the Sun. In earthly... ... Natural science. encyclopedic Dictionary
Light pressure- the pressure exerted by light on the illuminated surface. Plays an important role in cosmic processes (formation of comet tails, equilibrium of large stars). D.S. predicted in 1619 in Germany. astronomer I. Kepler. (1571 1630) and experimentally... ... Astronomical Dictionary
