Today we will talk about Lebedev’s experience in proving the pressure of photons of light. We will reveal the importance of this discovery and the premises that led to it.
Knowledge is curiosity
There are two points of view about the phenomenon of curiosity. One is expressed by the saying "a curious Barbara in the bazaar was torn off her nose", and the other - by the saying "curiosity is not a vice." This paradox is easily resolved if we distinguish between areas in which interest is not welcomed or, conversely, needed.
Johannes Kepler was not born to become a scientist: his father fought, and his mother kept a tavern. But he possessed extraordinary abilities and, of course, was curious. In addition, Kepler suffered from a serious visual impairment. But it was he who made the 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
Fifteen hundred years ago, mathematics and physics belonged to the "Art" section. Therefore, Copernicus was engaged in the mechanics of the motion of bodies (including celestial), and optics, and gravity. It was he who proved the existence of inertia. From the conclusions of this scientist, modern mechanics grew up, the concept of body interactions, the science of the exchange of speeds of objects in contact. Copernicus also developed a harmonious linear optics system.
He introduced concepts such as:
- "refraction of light";
- "refraction";
- "Optical axis";
- "Full internal reflection";
- "illumination".
And his research ultimately proved the wave nature of light and led to Lebedev’s experiment in measuring photon pressure.
The quantum properties of light
To begin with, it is worth determining the essence of light and telling what it is. A photon is a quantum of an electromagnetic field. It is a package of energy that moves in space as a whole. One cannot “bite off” a little energy from a photon, but it can be converted. For example, if light is absorbed by a substance, then its energy inside the body can undergo changes and radiate back a photon with a different energy. But this formally will not be the same quantum of light that is absorbed.
An example of this is a solid metal ball. If a piece of matter is torn from its surface, the shape will change, cease to be spherical. But if you melt the whole object, take a little liquid metal, and then create a smaller ball from the remains, then this will again be a sphere, but another, not the same as before.
Wave properties of light
Photons have the properties of a wave. Basic parameters are:
- wavelength (characterizes space);
- frequency (characterizes time);
- amplitude (characterizes the strength of the oscillation).
However, as a quantum of the electromagnetic field, the photon also has a propagation direction (denoted as a wave vector). In addition, the amplitude vector is able to rotate around the wave vector and create polarization of the wave. 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 case, the search for a law according to which one quantity turns into another is 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, light pressure must exist.
Experience structure
However, since the photons are very small, then their mass should be small. To build a device that could determine it with sufficient accuracy was a difficult technical task. The Russian scientist Petr Nikolaevich Lebedev was the first to deal with it.
The experience itself was based on the design of the balance, which determined the moment of torsion. A crossbar was hung on a silver thread. The same thin plates of various materials were attached to its ends. Most often, Lebedev used metals (silver, gold, nickel), 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, and the other remained in the shade. Lebedev’s experience proved that lighting on one side causes the balance to spin. According to the angle of deviation, the scientist judged the power of light.
Difficulties of experience
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, and work with glass, and polish surfaces. In fact, knowledge was obtained manually. There were then still no large corporations that would produce the necessary equipment in hundreds of pieces. Lebedev's device was created manually, so the scientist encountered a number of difficulties.
The vacuum at that time was not even average. The scientist pumped air from under a glass cover 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 transmission) 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, there would be no molecules whose Brownian motion on the illuminated side would be stronger.
The sensitivity of the angle of deviation left much to be desired. Modern helical determinants can measure the angle to millionths of a radian. At the beginning of the nineteenth century, the scale could be seen with the naked eye. The technique of that time could not provide the identical weight and size of the plates. This, in turn, did not allow the uniform distribution of 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 part was heated more for some reason (this is called a temperature gradient), then the wire could begin to twist even without light pressure. Despite the fact that the Lebedev device was quite simple and gave a large error, the fact of the momentum transfer by light photons was confirmed.
Plate Lighting Form
The previous section lists the many technical difficulties that existed in the experiment, but did not affect the main thing - the light. Purely theoretically, we imagine that a beam of monochromatic rays that are strictly parallel to each other falls on a plate. But at the beginning of the twentieth century, the sun, candles and simple incandescent lamps were the source of light. To make the beam of rays parallel, complex lens systems were built. And in this case, the most important factor was the light intensity curve of the source.
Physics classes often say that rays come from one point. But real light generators have certain sizes. In addition, the middle of the 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 light intensity curve.
Bloody moon and partial eclipse
Vampire novels abound in the terrible transformations that happen to people and nature into a bloody moon. But it does not say that one should not be afraid of this phenomenon. 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 edges of the visible disk reach the surface of the planet. Thus, when the Moon enters the penumbra of the Earth, then it does not completely obscure, but rather turns red. The atmosphere of the planet is also to blame for this shade: it absorbs all visible wavelengths, except for orange. Remember, the Sun also turns red at sunset, and all precisely because it passes through a thicker layer of the atmosphere.
How is the ozone layer of the Earth’s atmosphere created?
A meticulous reader may ask: “What does the pressure of light have to do with it, Lebedev’s experiments?” The chemical effect of light, by the way, is also due to the fact that the photon carries an impulse. Namely, this phenomenon is responsible for some layers of the planet’s atmosphere.
As you know, our airy ocean mainly absorbs the ultraviolet component of sunlight. Moreover, life in a certain form would be impossible, bathe the rocky surface of the earth in ultraviolet light. But at an altitude of about 100 km, the atmosphere is not yet so thick as to absorb everything. And ultraviolet gets the opportunity to interact with oxygen directly. It breaks down O 2 molecules into free atoms and promotes their combination into another modification - O 3 . In its pure form, this gas is deadly. That is why it is used to disinfect air, water, clothing. But as part of the earth’s atmosphere, it protects all life from the effects of harmful radiation, because the ozone layer very effectively absorbs quanta of electromagnetic fields with energy above the visible spectrum.