It is noted that there are some fine lines, that is, some colors are missing from the spectrum. This phenomenon was first noticed by the Bavarian physicist Josef von Fraunhofer on March 10, 1814. He described his observation as follows: “I found with the apparatus innumerable strong and weak vertical lines, darker than the rest of the colours, some of them appearing to be almost black.”
Physicist Josef von Fraunhofer discovered more than 500 of these dark bands in the solar spectrum. Thus, spectroscopy was born. However, it took a few years before scientists were able to explain the mysterious lines.
It was only in 1859 that the physicist Gustav Robert Kirchhoff, turning pure sodium salt into a colorless flame, brought light – literally – to the Fraunhofer mystery. The sodium in the fire produces exactly the lines missing in sunlight that scientists previously observed.
Kirchhoff concluded that there must be a large amount of sodium in the sun: “I concluded that the dark lines in the solar spectrum are caused by the presence of this element in the glowing atmosphere of the sun. And that it causes the formation of light lines in the spectrum of the flame.”
Flame test
Hidden behind this assumption is an important discovery: pure elements, when they are in combustion, emit light formed into distinct lines. Thus, light provides information about the chemical composition of the objects that emit it. This is the principle of chemical spectroscopy. This principle is still used by chemistry students to this day in what is called a flame test.
Chemist Michael Stefan explains how it works: “First you have to light a ceramic rod in a Bunsen burner. At first it stays colorless. Then you put out the fire and take a sample. Then you have a beautiful yellow-orange flame. This indicates the presence of sodium, which is the same color we know from street lamps. A very intense color that lasts for a relatively long time before it disappears, so the presence of sodium can be easily identified.”
Demonstrating through a flame is so effective that it can be seen with the naked eye, even when sodium is present in minimal amounts. The most difficult thing is to prove the existence of the barium element. In this case, chemists have to resort to trickery.
“The sample turns green. So there’s the barium. Magnesium powder is used to amplify the effect. Magnesium burns quickly without emitting color – previously used in camera flashes. Magnesium is then added to the flame, which burns and emits a bright green light. This indicates on the presence of barium in the sample.
Know the stars
Through similar experiments many chemical elements were discovered. For example, chemist Robert Wilhelm Bunsen extracted seven grams of a solid from 44,000 liters of mineral water. When Bunsen burned such a substance, he was able to establish the existence of cesium, a hitherto unknown element.
Through spectral analysis, scientists have found new elements outside the planet as well. In 1868, a line was identified in the solar spectrum that had not been seen on Earth before. Thus, the researchers classified a new element and named it helium. Only decades later, the noble gas helium was also discovered on our planet.
Author: Ute Hänsler / mas
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