Line spectra - this is perhaps one of the important topics that are considered in the 8th grade physics course in the optics section. It is important because it allows us to understand the atomic structure, as well as to use this knowledge to study our Universe. Let's consider this issue in the article.
The concept of electromagnetic spectra
First of all, let's explain what the article will be about. Everyone knows that the sunlight we see is electromagnetic waves. Any wave is characterized by two important parameters - its length and frequency (its third, no less important property is the amplitude, which reflects the intensity of the radiation).
In the case of electromagnetic radiation, both parameters are related in the following equation: λν=c, where the Greek letters λ (lambda) and ν (nu) usually denote the wavelength and its frequency, respectively, and c is the speed of light. Since the latter is a constant value for vacuum, the length and frequency of electromagnetic waves are inversely proportional to each other.
The electromagnetic spectrum in physics is acceptedname the set of different wavelengths (frequencies) that are emitted by the corresponding radiation source. If the substance absorbs, but does not emit waves, then one speaks of an adsorption or absorption spectrum.
What are electromagnetic spectra?
In general, there are two criteria for their classification:
- By radiation frequency.
- According to the frequency distribution method.
We will not dwell on the consideration of the 1st type of classification in this article. Here we will only briefly say that there are electromagnetic waves of high frequencies, which are called gamma radiation (>1020 Hz) and X-ray (1018-10 19 Hz). The ultraviolet spectrum is already lower frequencies (1015-1017 Hz). The visible or optical spectrum lies in the frequency range 1014 Hz, which corresponds to a set of lengths from 400 µm to 700 µm (some people are able to see a little "wider": from 380 µm to 780 µm). Lower frequencies correspond to the infrared or thermal spectrum, as well as radio waves, which can already be several kilometers long.
Later in the article, we will take a closer look at the 2nd type of classification, which is noted in the list above.
Line and continuous emission spectra
Absolutely any substance, if heated, will emit electromagnetic waves. What frequencies and wavelengths will they be? The answer to this question depends on the state of aggregation of the substance under study.
Liquid and solids emit, as a rule, a continuous set of frequencies, that is, the difference between them is so small that we can talk about a continuous spectrum of radiation. In turn, if an atomic gas having low pressures is heated, it will begin to "glow", emitting strictly defined wavelengths. If the latter are developed on photographic film, then they will be narrow lines, each of which is responsible for a specific frequency (wavelength). Therefore, this type of radiation was called the line emission spectrum.
Between line and continuous there is an intermediate type of spectrum, which usually emits a molecular rather than an atomic gas. This type is isolated bands, each of which, when examined in detail, consists of separate narrow lines.
Line absorption spectrum
All that was said in the previous paragraph referred to the radiation of waves by matter. But it also has absorbency. Let's carry out the usual experiment: let's take a cold discharged atomic gas (for example, argon or neon) and let white light from an incandescent lamp pass through it. After that, we analyze the light flux passing through the gas. It turns out that if this flux is decomposed into individual frequencies (this can be done using a prism), then black bands appear in the observed continuous spectrum, which indicate that these frequencies were absorbed by the gas. In this case, one speaks of a line absorption spectrum.
In the middle of the XIX century. German scientist named GustavKirchhoff discovered a very interesting property: he noticed that the places where black lines appear on the continuous spectrum correspond exactly to the frequencies of the radiation of a given substance. Currently, this feature is called Kirchhoff's law.
Balmer, Liman and Pashen series
Since the end of the 19th century, physicists around the world have sought to understand what the line spectra of radiation are. It was found that each atom of a given chemical element under any conditions exhibits the same emissivity, that is, it emits electromagnetic waves of only specific frequencies.
The first detailed studies of this issue were carried out by the Swiss physicist Balmer. In his experiments, he used hydrogen gas heated to high temperatures. Since the hydrogen atom is the simplest among all known chemical elements, it is easiest to study the features of the radiation spectrum on it. Balmer got an amazing result, which he wrote down as the following formula:
1/λ=RH(1/4-1/n2).
Here λ is the length of the emitted wave, RH - some constant value, which for hydrogen is equal to 1, 097107m -1, n is an integer starting from 3, i.e. 3, 4, 5 etc.
All lengths λ, which are obtained from this formula, lie within the optical spectrum visible to humans. This series of λ values for hydrogen is called the spectrumBalmer.
Subsequently, using the appropriate equipment, the American scientist Theodore Liman discovered the ultraviolet hydrogen spectrum, which he described with a formula similar to Balmer's:
1/λ=RH(1/1-1/n2).
Finally, another German physicist, Friedrich Paschen, obtained a formula for the emission of hydrogen in the infrared region:
1/λ=RH(1/9-1/n2).
Nevertheless, only the development of quantum mechanics in the 1920s could explain these formulas.
Rutherford, Bohr and the atomic model
In the first decade of the 20th century, Ernest Rutherford (British physicist of New Zealand origin) conducted many experiments to study the radioactivity of various chemical elements. Thanks to these studies, the first model of the atom was born. Rutherford believed that this "grain" of matter consists of an electrically positive nucleus and negative electrons rotating in its orbits. Coulomb forces explain why the atom "does not fall apart", and the centrifugal forces acting on electrons are the reason why the latter do not fall into the nucleus.
Everything seems to be logical in this model, except for one but. The fact is that when moving along a curvilinear trajectory, any charged particle must radiate electromagnetic waves. But in the case of a stable atom, this effect is not observed. Then it turns out that the model itself is wrong?
The necessary amendments were made to itanother physicist is the Dane Niels Bohr. These amendments are now known as his postulates. Bohr introduced two propositions into Rutherford's model:
- electrons move in stationary orbits in an atom, while they do not emit or absorb photons;
- the process of radiation (absorption) occurs only when an electron moves from one orbit to another.
What are stationary Bohr orbits, we will consider in the next paragraph.
Quantization of energy levels
The stationary orbits of an electron in an atom, which Bohr first spoke about, are stable quantum states of this particle-wave. These states are characterized by a certain energy. The latter means that the electron in the atom is in some energy "well". He can get into another "pit" if he receives additional energy from the outside in the form of a photon.
In the line absorption and emission spectra for hydrogen, the formulas of which are given above, you can see that the first term in brackets is a number of the form 1/m2, where m=1, 2, 3.. is an integer. It reflects the number of the stationary orbit to which the electron passes from a higher energy level n.
How do they study spectra in the visible range?
It has already been said above that glass prisms are used for this. This was first done by Isaac Newton in 1666, when he decomposed visible light into a set of rainbow colors. The reason forwhich this effect is observed is the dependence of the refractive index on the wavelength. For example, blue light (short wavelength) is more refracted than red light (long wavelength).
Note that in the general case, when a beam of electromagnetic waves moves in any material medium, the high-frequency components of this beam are always refracted and scattered more strongly than the low-frequency ones. A prime example is the blue color of the sky.
Lens optics and visible spectrum
When working with lenses, sunlight is often used. Since it is a continuous spectrum, when passing through the lens, its frequencies are refracted differently. As a result, the optical device is unable to collect all the light at one point, and iridescent shades appear. This effect is known as chromatic aberration.
The indicated problem of lens optics is partially solved by using a combination of optical glasses in appropriate instruments (microscopes, telescopes).
