Today we will talk about Lebedev's experiment in proving the pressure of light photons. We will reveal the importance of this discovery and the background that led to it.
Knowledge is curiosity
There are two points of view about the phenomenon of curiosity. One is expressed by the saying "curious Varvara's nose was torn off at the market", and the other - by the saying "curiosity is not a vice." This paradox is easily resolved if one distinguishes between areas in which interest is not welcome or, on the contrary, is needed.
Johannes Kepler was not born to become a scientist: his father fought in the war, and his mother kept a tavern. But he had extraordinary abilities and, of course, was curious. In addition, Kepler suffered from a severe visual impairment. But it was he who made discoveries, thanks to which science and the whole world are where they are now. Johannes Kepler is famous for clarifying the planetary system of Copernicus, but today we will talk about other accomplishments of the scientist.
Inertia and Wavelength: A Medieval Legacy
Fifty thousand years ago, mathematics and physics belonged to the "Art" section. Therefore, Copernicus was engaged in the mechanics of the movement of bodies (including celestial ones), and optics, and gravity. It was he who proved the existence of inertia. From the conclusionsThis scientist grew modern mechanics, the concept of the interactions of bodies, the science of the exchange of velocities of contacting objects. Copernicus also developed a harmonious system of linear optics.
He introduced concepts such as:
- "refraction of light";
- "refraction";
- "optical axis";
- "total internal reflection";
- "illumination".
And his research eventually proved the wave nature of light and led to Lebedev's experiment in measuring the pressure of photons.
Quantum properties of light
First of all, it is worth defining the essence of light and talking about what it is. A photon is a quantum of an electromagnetic field. It is a package of energy that moves through space as a whole. You can't "bite off" a bit of energy from a photon, but it can be transformed. For example, if light is absorbed by a substance, then inside the body its energy is able to undergo changes and emit back a photon with a different energy. But formally, this will not be the same quantum of light that was absorbed.
An example of this would be a solid metal ball. If a piece of matter is torn from its surface, then the shape will change, it will cease to be spherical. But if you melt the whole object, take some liquid metal, and then create a smaller ball from the remnants, then it will again be a sphere, but different, not the same as before.
Wave properties of light
Photons have the properties of a wave. Basic parameters are:
- wavelength (characterizes space);
- frequency (characterizestime);
- amplitude (characterizes the strength of the oscillation).
However, as a quantum of an electromagnetic field, a photon also has a direction of propagation (denoted as a wave vector). In addition, the amplitude vector is able to rotate around the wave vector and create wave polarization. With the simultaneous emission of several photons, the phase, or rather the phase difference, also becomes an important factor. Recall that the phase is that part of the oscillation that the wave front has at a particular moment in time (rise, maximum, descent or minimum).
Mass and energy
As Einstein wittily proved, mass is energy. But in each specific case, the search for a law according to which one value turns into another can be difficult. All of the above wave characteristics of light are closely related to energy. Namely: increasing the wavelength and decreasing the frequency means less energy. But since there is energy, then the photon must have mass, therefore, there must be light pressure.
Experience structure
However, since photons are very small, their mass should also be small. To build a device that could determine it with sufficient accuracy was a difficult technical task. Russian scientist Lebedev Petr Nikolaevich was the first to cope with it.
The experience itself was based on the design of the weights that determined the moment of torsion. A crossbar was hung on a silver thread. Attached to its ends were identical thin plates of variousmaterials. Most often, metals (silver, gold, nickel) were used in Lebedev's experiment, but there was also mica. The whole structure was placed in a glass vessel, in which a vacuum was created. After that, one plate was illuminated, while the other remained in shadow. Lebedev's experience proved that illumination of one side leads to the fact that the scales begin to spin. According to the angle of deviation, the scientist judged the strength of light.
Experience difficulties
At the beginning of the twentieth century, it was difficult to set up a sufficiently accurate experiment. Every physicist knew how to create a vacuum, work with glass, and polish surfaces. In fact, knowledge was obtained manually. At that time, there were no large corporations that would produce the necessary equipment in hundreds of pieces. Lebedev's device was created by hand, so the scientist faced a number of difficulties.
The vacuum at that time was not even average. The scientist pumped out air from under a glass cap with a special pump. But the experiment took place at best in a rarefied atmosphere. It was difficult to separate the pressure of light (impulse transfer) from the heating of the illuminated side of the device: the main obstacle was the presence of gas. If the experiment were carried out in a deep vacuum, then there would be no molecules whose Brownian motion on the illuminated side would be stronger.
The sensitivity of the deflection angle left much to be desired. Modern screw finders can measure angles down to millionths of a radian. At the beginning of the nineteenth century, the scale could be seen with the naked eye. Techniquetime could not provide identical weight and size of the records. This, in turn, made it impossible to evenly distribute the mass, which also created difficulties in determining the torque.
The insulation and structure of the thread greatly affects the result. If one end of the metal piece was heated more for some reason (this is called a temperature gradient), then the wire could begin to twist without light pressure. Despite the fact that Lebedev's device was quite simple and gave a large error, the fact of momentum transfer by photons of light was confirmed.
Shape of lighting plates
The previous section listed many technical difficulties that existed in the experiment, but did not affect the main thing - light. Purely theoretically, we imagine that a beam of monochromatic rays falls on the plate, which are strictly parallel to each other. But at the beginning of the twentieth century, the source of light was the sun, candles and simple incandescent lamps. To make the beam of rays parallel, complex lens systems were built. And in this case, the luminous intensity curve of the source was the most important factor.
In physics class it is often said that rays come from one point. But real light generators have certain dimensions. Also, the middle of a filament can emit more photons than the edges. As a result, the lamp illuminates some areas around it better than others. The line that goes around the whole space with the same illumination from a given source is called the luminous intensity curve.
Blood moon and partial eclipse
Vampire novels are replete with terrible transformations that happen to people and nature in the blood moon. But it does not say that this phenomenon should not be feared. Because it is the result of the large size of the Sun. The diameter of our central star is approximately 110 Earth diameters. At the same time, photons emitted from both one and the other edge of the visible disk reach the surface of the planet. Thus, when the Moon falls into the penumbra of the Earth, it is not completely obscured, but, as it were, turns red. The atmosphere of the planet is also to blame for this shade: it absorbs all visible wavelengths, except for orange ones. Remember, the Sun also turns red at sunset, and all precisely because it passes through a thicker layer of the atmosphere.
How is the Earth's ozone layer created?
A meticulous reader may ask: "What does the pressure of light have to do with Lebedev's experiments?" The chemical effect of light, by the way, is also due to the fact that the photon carries momentum. Namely, this phenomenon is responsible for some layers of the planet's atmosphere.
As you know, our air ocean mainly absorbs the ultraviolet component of sunlight. Moreover, life in a known form would be impossible if the rocky surface of the earth were bathed in ultraviolet light. But at an altitude of about 100 km, the atmosphere is not yet thick enough to absorb everything. And ultraviolet gets the opportunity to interact with oxygen directly. It breaks the molecules O2 intofree atoms and promotes their combination into another modification - O3. In its pure form, this gas is deadly. That is why it is used to disinfect air, water, clothes. But as part of the earth's atmosphere, it protects all living things from the effects of harmful radiation, because the ozone layer very effectively absorbs quanta of the electromagnetic field with energies above the visible spectrum.
