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MOST IMPORTANT SCIENTIFIC DISCOVERIES
Spectral analysis. History and essence of scientific discovery
Directory / The most important scientific discoveries When a ray of the sun passes through a prism, a spectrum appears on the screen behind it. For two hundred years, we have become accustomed to this phenomenon. If you do not look closely, it seems that there are no sharp boundaries between the individual parts of the spectrum: red continuously turns into orange, orange into yellow, etc. More carefully than others in 1802, the English physician and chemist William Hyde Wollaston (1766–1828) examined the spectrum. Wollaston discovered several sharp dark lines that, without visible order, intersected the spectrum of the Sun in different places. The scientist did not attach much importance to these lines. He believed that their appearance is caused either by the characteristics of the prism, or by the characteristics of the light source, or by some other secondary causes. The lines themselves were of interest to him only because they separated the colored bands of the spectrum from each other. Later, these dark lines were called Fraunhofer lines, perpetuating the name of their real researcher. Joseph Fraunhofer (1787-1826) at the age of 11, after the death of his parents, went to study with a grinding master. Due to work, there was little time left for school. Until the age of 14, Joseph could neither read nor write. But there was no happiness, but misfortune helped. One day the owner's house collapsed. When Joseph was removed from the rubble, the crown prince drove by. He took pity on the young man and handed him a considerable amount of money. The young man had enough money to buy himself a grinding machine and start studying. Fraunhofer in the provincial town of Benediktbeiren learned to grind optical glasses. In his preface to the collected works of Fraunhofer, E. Lommel summed up his contribution to practical optics in the following way. "Thanks to the introduction of his new and improved methods, mechanisms and measuring instruments for rotating and polishing lenses ... he managed to obtain sufficiently large samples of flint glass and crown glass without any veins. Of particular importance was the method he found for accurately determining the shape of lenses, which completely changed the direction development of practical optics and brought the achromatic telescope to such perfection, which could not even be dreamed of before. To make accurate measurements of the dispersion of light in prisms, Fraunhofer used a candle or lamp as a light source. At the same time, he discovered a bright yellow line in the spectrum, now known as the yellow line of sodium. It was soon established that this line is always in the same place in the spectrum, so that it is very convenient to use it for accurate measurements of refractive indices. After that, says Fraunhofer in his first work of 1815: "... I decided to find out if it was possible to see such a luminous line in the solar spectrum. And with the help of a telescope I found not one line, but an extremely large number of vertical lines, sharp and weak , which, however, turned out to be darker than the rest of the spectrum, and some of them appeared almost completely black." In total, he counted them there 574. Fraunhofer gave names and indicated their exact location in the spectrum. It was found that the position of the dark lines was strictly unchanged, in particular, a sharp double line always appeared in the same place in the yellow part of the spectrum. Fraunhofer called it the line O. The scientist also discovered that in the spectrum of the flame of an alcohol lamp at the same place as the dark line O in the spectrum of the Sun, there is always a bright double yellow line. It was not until many years later that the significance of this discovery became clear. Continuing his studies of dark lines in the spectrum of the Sun, Fraunhofer realized the main thing: their cause is not in an optical illusion, but in the very nature of sunlight. As a result of further observations, he found similar lines in the spectrum of Venus and Sirius. One discovery of Fraunhofer, as it turned out later, turned out to be especially important. We are talking about the observation of the double D-line. In 1814, when the scientist published his research, this observation was not paid much attention. However, 43 years later, William Swan (1828–1914) found that the double yellow O line in the spectrum of a spirit lamp flame appears in the presence of sodium metal. Alas, like many before him, Swan did not realize the significance of this fact. He never said the decisive words: "This line belongs to the sodium metal." In 1859, two scientists came to this simple and important idea: Gustav Robert Kirchhoff (1824–1887) and Robert Wilhelm Bunsen (1811–1899). In the Heidelberg University Laboratory they set up the following experiment. Before them, either only a ray of the Sun was passed through a prism, or only light from a spirit lamp. Scientists decided to skip them at the same time. As a result, they discovered a phenomenon, which L.I. Ponomarev: “If only the Sun’s beam fell on the prism, then on the scale of the spectroscope they saw the Sun’s spectrum with a dark line O in its usual place. The dark line still remained in place even when the researchers placed a burning spirit lamp in the path of the beam. But when they placed a screen in the path of a sunbeam and illuminated the prism only with the light of an alcohol lamp, a bright yellow sodium line O clearly appeared in place of the dark line O. Kirchhoff and Bunsen removed the screen - the line O again became dark. Then they replaced the sun's ray with light from a hot body - the result was always the same: a dark one appeared in place of a bright yellow line. That is, the flame of the spirit lamp always absorbed the rays that it itself emitted. To understand why this event excited the two professors, let us follow their reasoning. The bright yellow O line in the spectrum of the spirit lamp flame appears in the presence of sodium. In the spectrum of the Sun, a dark line of unknown nature is located at the same place. The spectrum of the beam from any hot body is continuous, and there are no dark lines in it. However, if such a beam is passed through the flame of an alcohol lamp, then its spectrum is no different from the spectrum of the Sun - it also has a dark line in the same place. But we already almost know the nature of this dark line, in any case, we can guess that it belongs to sodium. Therefore, depending on the conditions of observation, the sodium O line can be either bright yellow or dark on a yellow background. But in both cases, the presence of this line (it doesn't matter which one - yellow or dark!) Means that there is sodium in the flame of the spirit lamp. And since such a line in the spectrum of the flame of an alcohol lamp in transmitted light coincides with the dark line O in the spectrum of the Sun, it means that there is sodium on the Sun. Moreover, it is located in the gaseous outer cloud, which is illuminated from the inside by the hot core of the Sun. A short note of two pages, written by Kirchhoff in 1859, contained four discoveries at once: - each element has its own line spectrum, which means a strictly defined set of lines; - such lines can be used to analyze the composition of substances not only on Earth, but also on stars; - The sun consists of a hot core and a relatively cold atmosphere of hot gases; The Sun contains the element sodium. The first three propositions were soon confirmed, in particular, the hypothesis about the structure of the Sun. The expedition of the French Academy of Sciences in 1868, led by the astronomer Jansen, visited India. She discovered that during a total solar eclipse, at the moment when its hot core is covered by the Moon's shadow and only the corona shines, all the dark lines in the Sun's spectrum flash with bright light. Kirghof and Bunsen not only brilliantly confirmed the second position, but also used it to discover two new elements: rubidium and cesium. This is how spectral analysis was born, with the help of which it is now possible to find out the chemical composition of distant galaxies, measure the temperature and rotation speed of stars, and much more. Later, electric voltage was most often used to bring elements into an excited state. Under the influence of voltage, the elements emit light characterized by certain wavelengths, that is, having a certain color. This light is split in a spectral apparatus (spectroscope), the main part of which is a glass or quartz prism. In this case, a strip is formed, consisting of separate lines, each of which is characteristic of a certain element. For example, it was previously known that the mineral kleveite, when heated, releases a gas similar to nitrogen. This gas, when studied with a spectroscope, turned out to be a new, as yet unknown noble gas. When electrically excited, it emitted lines that had previously been detected when analyzing the rays of the sun with a spectroscope. It was a peculiar case when an element previously discovered on the Sun was discovered by Ramsay on Earth as well. He was given the name helium, from the Greek word "helios" - the sun. Today, two types of spectra are known: continuous (or thermal) and line. As Ponomarev writes, “the thermal spectrum contains all wavelengths, it is emitted when solids are heated and does not depend on their nature. The line spectrum consists of a set of individual sharp lines, occurs when gases and vapors are heated (when interactions between atoms are small), and - what is especially important - this set of lines is unique for any element. Moreover, the line spectra of elements do not depend on the type of chemical compounds composed of these elements. Therefore, their cause must be sought in the properties of atoms. The fact that the elements are uniquely and completely determined by the type of the line spectrum was soon recognized by everyone, but the fact that the same spectrum characterizes an individual atom was not realized immediately, but only in 1874, thanks to the works of the famous English astrophysicist Norman Lockyer (1836-1920). And when they realized, they immediately came to the inevitable conclusion: since the line spectrum arises inside a single atom, then the atom must have a structure, that is, have constituent parts! Author: Samin D.K.
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