Today we will devote a conversation to such a phenomenon as light pressure. Consider the premises of the discovery and the consequences for science.
Light and color
The mystery of human abilities has worried 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 around 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.general. The corpuscular-wave theory 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 has now explained it in sufficient detail.
Wavelength and energy
Slightly above the concept of "wave energy" was mentioned. 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. The experiments of Lebedev, a researcher from Russia, convincingly proved this amazing fact.
Lebedev's experiment
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 a very rarefied atmosphere under the glass cover. And alternately illuminated one plate, leaving the other in shadow. The amount of light directed at the surfaces was predetermined. Based on the angle of deflection, Lebedev determined what impulse transmitted the light to the plates.
Formulas for determining the pressure of electromagnetic radiation at normal beam incidence
Let's first explain what a "normal fall" is? 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 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 has already guessed that such an ideal combination of factors does not exist. Even if the ideal surface is not taken into account, it is rather difficult to organize the incidence of light strictly perpendicular.
Formulas fordetermining 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 that 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’ are unit vectors that show the direction of the incident and reflected beam of light (they set the directions in which the acting forces should be added);
- ϴ - angle to the normal at which the light ray falls (and, accordingly, is reflected, since the surface is mirrored).
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 student 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 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 a lotinteresting applications:
- 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.
- Production of high-energy protons (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 correct 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 also potential opportunities that have not yet been realized, because the technology of mankind has not yet reached the required level. Among them:
- Solar sail. With its help, it would be possible to move rather 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.
- Photonic engine. This technology, perhaps, will allow a person to overcome the attraction of his native star and fly to other worlds. The difference from a solar sail is that an artificially created device, for example, a thermonuclear one, will generate solar pulses.engine.
