Today we will devote the conversation to such a phenomenon as the pressure of light. Consider the prerequisites of discovery and effect for science.
Light and color
The mystery of human ability has worried people since ancient times. How does the eye see? Why are there colors? What is the reason that the world is the way we feel it? How far is a person able to see? Experiments with the decomposition of the sun's ray into the spectrum were carried out by Newton in the 17th century. He laid a rigorous mathematical foundation in a series of disparate facts that at that time were known about the world. And Newtonian theory predicted a lot: for example, discoveries that were explained only by quantum physics (the deflection of light in a gravitational field). But the exact nature of light, physics of that time did not know and did not understand.
Wave or particle
Since scientists around the world began to penetrate into the essence of the world, a debate has been going on: what is radiation, wave or 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 two versions into one general. The particle-wave theory claims that any microparticle, including a photon, has both wave and particle properties. 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. Such is the essence of the concept of "light." Physics these days 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 tricky 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 dark and calm bowels of metals and gases to emit light. The photon momentum, a direct consequence of the presence of mass, can be determined using light pressure. The experiments of Lebedev, a researcher from Russia, convincingly proved this amazing fact.
Lebedev Experience
The Russian scientist Petr Nikolaevich Lebedev in 1899 made the following experiment. On a thin silver thread he hung the bar. The scientist attached two plates of the same substance to the ends of the crossbar. It was silver foil, and gold, and even mica. Thus, peculiar scales were created. Only they did not measure the weight of the load, which presses on top, but the load, which presses on the side of 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 under the cover he created a vacuum. But at that time it was impossible to achieve even an average vacuum. So we’ll say that he created a very rarefied atmosphere under a glass lid . And alternately lit one plate, leaving another in the shade. The amount of light directed to the surface was set in advance. From the angle of deviation, Lebedev determined which impulse transmitted 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 falls on the surface normally if it is directed strictly perpendicular to the surface. This imposes limitations on the problem: the surface must be perfectly smooth, and the radiation beam is directed very precisely. In this case, the light pressure is calculated by the formula :
p = (1-k + ρ) * I / c,
Where
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 already guessed that such an ideal combination of factors does not exist. Even if you do not take into account the ideality of the surface, it is quite difficult to organize light incidence strictly perpendicularly.
Formulas for determining the pressure of electromagnetic radiation when it falls at an angle
The pressure of light on the mirror surface at an angle is calculated by another formula, which already contains the elements of the vectors:
p = ω ((1-k) i + ρi ') cos ϴ
The quantities p, i, i 'are vectors. Moreover, k and ρ, as in the previous formula, are the transmission and reflection coefficients, respectively. New values indicate the following:
- ω is the bulk density of radiation energy;
- i and i 'are unit vectors that show the direction of the incident and reflected light beam (they specify the directions along which the acting forces should be added);
- ϴ is the angle to the normal, under which a ray of light falls (and, accordingly, is reflected, since the surface is mirror).
We remind the reader that the normal is perpendicular to the surface, so if the angle of incidence of light to the surface is given in the problem, then ϴ is 90 degrees minus the specified value.
Application of the phenomenon of pressure of electromagnetic radiation
A schoolboy who studies physics, many formulas, concepts and phenomena seem boring. Because, as a rule, the teacher tells the theoretical aspects, but rarely can give examples of the benefits of certain phenomena. We will not blame school mentors for this: they are very limited by the program, during the lesson you need to tell extensive material and still have time to check the students' knowledge.
Nevertheless, the object of our study has many interesting applications:
- Now almost every student in the laboratory of his school can repeat Lebedev’s experience. But then the coincidence of experimental data with theoretical calculations was a real breakthrough. The experiment made for the first time with a 20 percent error allowed scientists around the world to develop a new branch of physics - quantum optics.
- Production of protons with high energy (for example, for irradiation of various substances) by accelerating thin films with a laser pulse.
- Taking into account the pressure of the electromagnetic radiation of the Sun on the surface of near-Earth objects, including satellites and space stations, allows you to adjust their orbit with greater accuracy and prevents these devices from falling to Earth.
The above applications exist now in the real world. But there are potential opportunities that have not yet been realized, because the technology of mankind has not yet reached the desired level. Among them:
- Solar sail. 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 desired position of the surface of the sail, the acceleration would be constant. In the absence of friction, it is enough to set speed and deliver goods to the desired point in the solar system.
- Photon engine. This technology, perhaps, will allow a person to overcome the attraction of his native star and fly to other worlds. The difference from the solar sail is that an artificially created device, for example, a thermonuclear engine, will generate solar pulses.