Linear spectra are 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 you to understand the atomic structure, as well as use this knowledge to study our universe. Let's consider this question in the article.
The concept of electromagnetic spectra
First of all, we will explain what will be discussed in the article. Everyone knows that the sunlight we see is electromagnetic waves. Any wave is characterized by two important parameters - its length and frequency (its third equally important property is the amplitude reflecting the radiation intensity).
In the case of electromagnetic radiation, both parameters are related into the following equality: λ * ν = c, where the Greek letters λ (lambda) and ν (nu) usually denote the wavelength and 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 called the set of different wavelengths (frequencies) that are emitted by the corresponding radiation source. If the substance absorbs, but does not emit waves, then they talk about the spectrum of adsorption or absorption.
What are the electromagnetic spectra?
In the general case, there are two criteria for their classification:
- By radiation frequency.
- By the method of frequency distribution.
In this article we will not dwell on the consideration of the 1st type of classification. Here we only briefly say that there are electromagnetic waves of high frequencies, which are called gamma radiation (> 10 20 Hz) and x-ray (10 18 -10 19 Hz). The ultraviolet spectrum is already lower frequencies (10 15 -10 17 Hz). The visible or optical spectrum lies in the frequency range 10 14 Hz, which corresponds to a set of lengths from 400 microns to 700 microns (some people are able to see a little "wider": from 380 microns to 780 microns). Lower frequencies correspond to the infrared or thermal spectrum, as well as radio waves, which can already reach several kilometers in length.
Further in the article, we will consider in detail the 2nd type of classification, which is noted in the list above.
Linear and continuous emission spectra
Absolutely any substance, if it is heated, will begin to emit electromagnetic waves. What frequencies and wavelengths will they be? The answer to this question depends on the state of aggregation of the test substance.
Liquid and solid bodies 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, then it will begin to “glow”, emitting strictly defined wavelengths. If the latter is shown on 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 atomic gas. This type is an isolated strip, each of which, when examined in detail, consists of separate narrow lines.
Line absorption spectrum
Everything said in the previous paragraph related to the emission of waves by matter. But it also has an absorption capacity. Let's conduct a usual experiment: take a cold, discharged atomic gas (for example, argon or neon) and pass through it white light from an incandescent lamp. After that, we analyze the light flux passing through the gas. It turns out that if this stream is decomposed into separate frequencies (this can be done using a prism), black bands appear in the observed continuous spectrum, which indicate that these frequencies were absorbed by the gas. In this case, they speak of a linear absorption spectrum.
In the middle of the XIX century. a German scientist named Gustav Kirchhoff discovered a very interesting property: he noticed that the places where black lines appear on the solid spectrum exactly correspond to the radiation frequencies of this substance. Currently, this feature is called the law of Kirchhoff.
Balmer, Estuary and Paschen Series
Since the end of the 19th century, physicists around the world have sought to understand what linear emission spectra 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 characteristics of the radiation spectrum on it. Balmer got an amazing result, which he wrote down as the following formula:
1 / λ = R H * (1 / 4-1 / n 2 ).
Here λ is the length of the emitted wave, R H is some constant value, which for hydrogen is 1,097 * 10 7 m -1 , n is an integer starting from 3, i.e. 3, 4, 5, etc.
All lengths λ that are obtained from this formula lie within the optical spectrum visible to humans. This series of λ values for hydrogen is called the Balmer spectrum.
Subsequently, using the appropriate equipment, the American scientist Theodore Liman discovered the ultraviolet hydrogen spectrum, which he described with a formula similar to Balmer:
1 / λ = R H * (1 / 1-1 / n 2 ).
Finally, another German physicist, Friedrich Paschen, obtained a formula for the emission of hydrogen in the infrared:
1 / λ = R H * (1 / 9-1 / n 2 ).
Nevertheless, only the development of quantum mechanics in the 1920s was able to explain these formulas.
Rutherford, Bohr and the Atomic Model
In the first decade of the 20th century, Ernest Rutherford (a British physicist of New Zealand origin) conducted many experiments to study the radioactivity of various chemical elements. Thanks to these studies, the first atom model was born. Rutherford believed that this "grain" of matter consists of a nucleus of electrically positive and negative electrons rotating in its orbits. The Coulomb forces explain why the atom "does not fall apart", and the centrifugal forces acting on the electrons are the reason why the latter do not fall on the nucleus.
Everything, it would seem, is logical in this model, with the exception of one but. The fact is that when moving along a curved path, any charged particle must emit electromagnetic waves. But in the case of a stable atom, this effect is not observed. Then it turns out that the model itself is incorrect?
The necessary amendments were made to it by another physicist - the Danes Niels Bohr. These amendments are now known as his postulates. Bohr introduced two provisions in the Rutherford model:
- the electrons move in stationary orbits in the atom, while they do not emit and do not absorb photons;
- The process of radiation (absorption) occurs only when the electron passes from one orbit to another.
What are the stationary Bohr orbits, we consider in the next paragraph.
Quantization of energy levels
The stationary orbits of the electron in the atom, about which Bohr first spoke, 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 kind of energy "well". He can get into another "hole" if he receives additional energy from the outside in the form of a photon.
In the linear absorption and emission spectra for hydrogen, the formulas of which are given above, it can be seen that the first term in brackets is a certain number of the form 1 / m 2 , 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 are spectra studied in the visible range?
It has already been said above that glass prisms are used for this. Isaac Newton did this for the first time in 1666, when he decomposed visible light into a set of rainbow colors. The reason this effect is observed is because the refractive index depends on the wavelength. So, the blue color (short waves) is refracted more strongly than red (long waves).
Note that in the general case, when a beam of electromagnetic waves moves in a material medium, the high-frequency components of this beam always refract and scatter more than the low-frequency ones. A vivid 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 in different ways. 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 using a combination of optical glasses in appropriate devices (microscopes, telescopes).