Emission and absorption of light by atoms. The origin of line spectra

This article presents the basic concepts needed to understand how atoms emit and absorb light. Also, the application of these phenomena is described here.

Smartphone and physics

A man who was born after 1990, his life without a variety of electronic devices can not imagine. The smartphone not only replaces the phone, but also allows you to monitor currency rates, make deals, call a taxi and even correspond with astronauts onboard the ISS through their applications. Accordingly, and all these digital assistants are perceived as a matter of course. The emission and absorption of light by atoms, thanks to which the era of reducing all kinds of devices became possible, will seem to such readers only a boring topic in physics lessons. But in this section of physics a lot of interesting and fascinating.

Theoretical prerequisites for the discovery of spectra

There is a saying: "Curiosity does not lead to good". But this expression is more likely to refer to the fact that it is better not to interfere with other people's relationships. If you show curiosity to the world around you, nothing will be wrong. At the end of the nineteenth century, people became aware of the nature of magnetism (it is well described in the system of Maxwell's equations). The next question that the scientists wanted to solve was the structure of matter. We must immediately clarify: for science is not valuable the very emission and absorption of light by atoms. Ruled spectra are a consequence of this phenomenon and a basis for studying the structure of substances.

The structure of the atom

Scientists even in ancient Greece supposed that marble consists of some indivisible bits, "atoms." And until the end of the nineteenth century, people thought that they were the smallest particles of matter. But Rutherford's experience of scattering heavy particles on a gold foil showed that the atom also has an internal structure. The heavy nucleus is in the center and charged positively, the light negative electrons rotate around it.

The paradoxes of an atom within the framework of Maxwell's theory

These data have brought to life several paradoxes: according to Maxwell's equations, any moving charged particle emits an electromagnetic field, therefore, it loses energy. Why, then, do not the electrons fall on the core, but continue to rotate? It was also not clear why each atom absorbs or emits photons of only a certain wavelength. The Bohr theory has made it possible to eliminate these discrepancies by introducing orbitals. According to the postulates of this theory, electrons can be around the nucleus only on these orbitals. The transition between two neighboring states is accompanied by either the emission or absorption of a quantum with a certain energy. The emission and absorption of light by atoms is due to this.

Wavelength, frequency, energy

For a more complete picture it is necessary to tell a little about photons. These are elementary particles, which have no rest mass. They exist only while they move through the environment. But the mass still have: bumping against the surface, they impart to it an impulse, which would have been impossible without the mass. They just turn their mass into energy, making the substance they hit and which they absorb, a little warmer. Bohr's theory does not explain this fact. The properties of a photon and the features of its behavior are described by quantum physics. So, a photon is both a wave and a particle with mass. A photon, and as a wave, has the following characteristics: length (λ), frequency (ν), energy (E). The longer the wavelength, the lower the frequency, and the lower the energy.

The spectrum of the atom

The atomic spectrum is formed in several stages.

1. The electron in the atom passes from the orbital 2 (with a higher energy) to the orbital 1 (with a lower energy).
2. Some amount of energy is released, which is formed as a quantum of light (hν).
3. This quantum is emitted into the surrounding space.

This is how the line spectrum of the atom is obtained. Why it is called that way, explains its form: when special devices "catch" the outgoing photons of light, a number of lines are fixed on the recording device. To separate photons of different wavelengths, the phenomenon of diffraction is used: waves with different frequencies have different refractive index, therefore, some deviate more strongly than others.

Properties of substances and spectra

The line spectrum of the substance is unique for each type of atom. That is, hydrogen emits one set of lines, and gold - the other. This fact is the basis for the application of spectrometry. Having received a spectrum of anything, you can understand what the substance consists of, how atoms are arranged in relation to each other. This method allows you to determine and the various properties of materials, which often uses chemistry and physics. Absorption and emission of light by atoms is one of the most common tools for studying the surrounding world.

Disadvantages of the method of emission spectra

Up to this point, it was more about how atoms radiate. But usually all the electrons are in their orbitals in a state of equilibrium, they have no reason to change over to other states. In order for a substance to emit something, it must first absorb energy. This is the drawback of the method that exploits the absorption and emission of light by an atom. Briefly say that the substance must first be heated or lighted before we get the spectrum. Questions will not arise if the scientist studies the stars, they are already shining due to their own internal processes. But if you want to study a piece of ore or food, then in order to get the spectrum, you actually need to burn it. This method does not always work.

Absorption spectra

Radiation and absorption of light by atoms as a method "works" in two directions. You can shine on the substance with broadband light (that is, in which there are photons of different wavelengths), and then see what wavelengths are absorbed. But this method is not always appropriate: it is necessary that the substance is transparent to the desired part of the electromagnetic scale.

Qualitative and quantitative analysis

It became clear: the spectra are unique for each substance. The reader could conclude that such an analysis is used only to determine what the material is made from. However, the spectra are much wider. With the help of special techniques for examining and recognizing the width and intensity of the resulting lines, it is possible to determine the number of atoms entering into a compound. And this indicator can be expressed in different units:

• In percent (for example, in this alloy contains 1% of aluminum);
• In moles (3 moles of common salt are dissolved in this liquid);
• In grams (in this sample there are 0.2 g of uranium and 0.4 grams of thorium).

Sometimes the analysis is mixed: qualitative and quantitative at the same time. But if earlier physicists memorized the position of lines by heart and assessed their shade using special tables, now all these programs do.

Application of spectra

We have already thoroughly analyzed what is the emission and absorption of light by atoms. Spectral analysis is used very widely. There is no area of human activity wherever the phenomenon we are considering is used. Here are some of them:

1. At the very beginning of the article, we talked about smartphones. Silicon semiconductor elements have become so small, including through the study of crystals with the help of spectral analysis.
2. In any event, the uniqueness of the electron shell of each atom makes it possible to determine which bullet was fired first, why the carcass of the car broke down or the tower crane fell, and what poison the person was poisoned and how long he stayed in the water.
3. Medicine uses spectral analysis for its own purposes most often with respect to body fluids, but it happens that this method is also applied to tissues.
4. Far galaxies, clouds of cosmic gas, planets in other people's stars - all this is studied with the help of light and its decomposition into spectra. Scientists recognize the composition of these objects, their speed and the processes that occur in them due to the fact that they can fix and analyze the photons that they emit or absorb.

Electromagnetic scale

Most of all we pay attention to visible light. But on the electromagnetic scale this segment is very small. What the human eye does not fix is much wider than the seven colors of the rainbow. Not only visible photons (λ = 380-780 nanometers), but also other quanta can be emitted and absorbed. The electromagnetic scale includes:

1. Radio waves (λ = 100 kilometers) transmit information over long distances. Because of the very long wavelength, their energy is very low. They are very easily absorbed.
2. Terahertz waves (λ = 1-0.1 millimeters) until recently were difficult to access. Earlier, their range was included in radio waves, but now this segment of the electromagnetic scale is allocated to a separate class.
3. Infrared waves (λ = 0.74-2000 micrometers) transfer heat. A bonfire, a lamp, the Sun emit them in abundance.

Visible light, we examined, so we will not write more about it.

Ultraviolet waves (λ = 10-400 nanometers) are lethal to humans in excess, but their deficiency causes irreversible processes. Our central star gives a lot of ultraviolet light, and the Earth's atmosphere holds up most of it.

X-ray and gamma quanta (λ <10 nanometers) have a common range, but differ in origin. To get them, you need to accelerate electrons or atoms to very high speeds. Laboratories of people are capable of this, but in nature such energies are found only inside the stars or in collisions of massive objects. An example of the latter process can be supernova explosions, the absorption of a star by a black hole, the encounter of two galaxies or a galaxy and a massive gas cloud.

Electromagnetic waves of all ranges, namely their ability to be emitted and absorbed by atoms, are used in human activity. Regardless of what the reader has chosen (or is only going to elect) as his life path, he will definitely encounter the results of spectral studies. The seller uses a modern payment terminal only because once the scientist researched the properties of substances and created a microchip. The agrarian fertilizes fields and collects large yields now only because once a geologist has discovered phosphorus in a piece of ore. The girl wears bright outfits only thanks to the invention of persistent chemical dyes.

But if the reader wishes to connect his life with the world of science, then it is necessary to study much more than the basic concepts of the process of radiation and absorption of light quanta in atoms.