This article provides the basic concepts needed to understand how atoms emit and absorb light. The application of these phenomena is also described here.
Smartphone and Physics
A man who was born later than 1990 cannot imagine his life without a variety of electronic devices. The smartphone not only replaces the phone, but also makes it possible to monitor exchange rates, make transactions, call a taxi and even correspond with astronauts on board the ISS through its applications. Accordingly, all these digital assistants are taken for granted. The emission and absorption of light by atoms, thanks to which the era of reduction of all kinds of devices became possible, such readers will find only a boring topic in physics lessons. But in this section of physics there is a lot of interesting and fascinating.
Theoretical premises for the discovery of spectra
There is a saying: "Curiosity will not bring to good." But this expression rather refers to the fact that it is better not to interfere in other people's relationships. If you show curiosity to the world around you, nothing bad will happen. At the end of the nineteenth century, people understood the nature of magnetism (it is well described in the Maxwell system of equations). The next question that scientists wanted to solve was the structure of matter. We must immediately clarify: for science, it is not the emission and absorption of light by atoms that is valuable. Linear spectra are a consequence of this phenomenon and the basis for studying the structure of substances.
Atom structure
Scientists in ancient Greece suggested that marble consists of some indivisible pieces, “atoms”. And until the end of the nineteenth century, people thought that these were the smallest particles of matter. But Rutherford's experience in scattering heavy particles on a gold foil showed that the atom also has an internal structure. A heavy nucleus is in the center and is positively charged, light negative electrons revolve around it.
Atom paradoxes in the framework of Maxwell's theory
These data 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 nucleus, but continue to spin? It was also not clear why each atom absorbs or emits photons of only a certain wavelength. Bohr's theory resolved these inconsistencies by introducing orbitals. According to the postulates of this theory, electrons can be around a nucleus only in these orbitals. The transition between two neighboring states is accompanied by either emission or absorption of a quantum with a certain energy. The emission and absorption of light by atoms occurs precisely because of this.
Wavelength, frequency, energy
For a more complete picture, you need to talk a little about photons. These are elementary particles that have no rest mass. They exist only as long as they move through the medium. But they still have mass: hitting the surface, they give it an impulse, which would be impossible without mass. They simply turn their mass into energy, making the substance that they hit and with which they are absorbed 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 like 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.
Atom spectrum
The atomic spectrum is formed in several stages.
- An electron in an atom transfers from orbital 2 (with a higher energy) to orbital 1 (with a lower energy).
- A certain amount of energy is released, which is formed as a quantum of light (hν).
- This quantum is radiated into the surrounding space.
Thus, a linear spectrum of the atom is obtained. Why it is called that way, its form explains: when special devices "catch" outgoing photons of light, a number of lines are recorded on the recording device. To separate photons of different wavelengths, the diffraction phenomenon is used: waves with different frequencies have different refractive indices, therefore, some deviate more strongly than others.
Properties of substances and spectra
The line spectrum of a substance is unique for each type of atom. That is, hydrogen when emitted will give one set of lines, and gold - another. This fact is the basis for the use of spectrometry. Having received a spectrum of anything, you can understand what the substance consists of, how atoms are located in it relative to each other. This method allows you to determine the various properties of materials, which often uses chemistry and physics. The absorption and emission of light by atoms is one of the most common tools for studying the surrounding world.
The disadvantages of the method of emission spectra
Up to this point, it was rather said about how atoms emit. But usually all electrons are in equilibrium in their orbitals, they have no reason to switch to other states. In order for a substance to radiate something, it must first absorb energy. This is the disadvantage of a method that exploits the absorption and emission of light by an atom. Briefly say that the substance must first be heated or lightened before we get the spectrum. No questions will arise if a scientist studies the stars, they are already shining thanks to their own internal processes. But if you want to study a piece of ore or a food product, then in order to obtain a spectrum it actually needs to be burned. This method is not always suitable.
Absorption spectra
Radiation and absorption of light by atoms as a method "works" in two directions. You can illuminate the substance with broadband light (that is, one in which photons of different wavelengths are present), and then see what wavelengths are absorbed. But this method is not always suitable: it is imperative 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: such an analysis is used only to determine what the material is made of. However, the possibilities of the spectra are much wider. Using special techniques for considering and recognizing the width and intensity of the resulting lines, one can establish the number of atoms entering the compound. Moreover, this indicator can be expressed in different units:
- in percent (for example, this alloy contains 1% aluminum);
- in moles (3 moles of sodium chloride are dissolved in this liquid);
- in grams (0.2 g of uranium and 0.4 grams of thorium are present in this sample).
Sometimes the analysis is mixed: qualitative and quantitative at the same time. But if earlier physicists memorized the position of the lines and estimated their shade using special tables, now all this is done by programs.
Spectrum Application
We have already examined in sufficient detail what is the emission and absorption of light by atoms. Spectral analysis is used very widely. There is not a single area of human activity wherever the phenomenon we are considering is used. Here is some of them:
- At the very beginning of the article we talked about smartphones. Silicon semiconductor elements have become so small, thanks in part to crystal studies using spectral analysis.
- In any incident, it is the uniqueness of the electron shell of each atom that allows you to determine which bullet was fired first, why the frame of the machine broke or the tower crane fell, how much poison the person poisoned, and how long he spent in the water.
- Medicine uses spectral analysis for its purposes most often in relation to body fluids, but it happens that this method is also applied to tissues.
- Distant galaxies, clouds of space gas, planets from alien 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 capture 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. The fact that 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:
- Radio waves (λ = 100 kilometers) transmit information over long distances. Due to the very long wavelength, their energy is very low. They are very easily absorbed.
- Terahertz waves (λ = 1-0.1 millimeters) until recently were difficult to access. Previously, their range was included in radio waves, but now this segment of the electromagnetic scale is allocated in a separate class.
- Infrared waves (λ = 0.74-2000 micrometers) carry heat. Bonfire, lamp, Sun radiate them in abundance.
We examined the visible light, so we won’t write about it in more detail.
Ultraviolet waves (λ = 10-400 nanometers) are lethal to humans in abundance, but their lack causes irreversible processes. Our central star gives a lot of ultraviolet radiation, and the Earth’s atmosphere delays most of it.
X-ray and gamma rays (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. Human laboratories are capable of this, but in nature such energies are found only inside stars or in collisions of massive objects. An example of the latter process is supernova explosions, the absorption of a star by a black hole, the encounter of two galaxies or galaxies and a massive cloud of gas.
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 just about to choose) as his path of life, he will certainly encounter the results of spectral studies. The seller uses a modern payment terminal only because once a scientist investigated the properties of substances and created a microchip. The landowner fertilizes the fields and is now harvesting large crops only because once a geologist 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 wants to connect his life with the world of science, then he will have to study much more than the basic concepts of the process of radiation and absorption of light quanta in atoms.