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During the Spring Festival, colorful and shaped fireworks bloom in the night sky, setting off a festive atmosphere. While enjoying the gorgeous fireworks, you might imagine that the scientific process of studying the composition of elements on the sun has something to do with the rich colors of the fireworks.

Gorgeous fireworks | Wikimedia Commons fireworks can show colorful colors, derived from specific metal compounds (or metal salts) added in the manufacturing process. These metal salts will react with flame color when burning, showing a special color. When the compounds with different metal ions burn, the color of the flame is also different, such as yellow light from sodium salt, red light from strontium salt, green light from barium salt and so on. Therefore, flame reaction has been used to qualitatively analyze the composition of compounds for a long time.

The flame reaction of elements can produce a special color | reference [5] so how can the flame reaction be associated with the composition of the sun? It starts with the physicists' study of sunlight.

In 1666, Issac Newton conducted an experiment on the dispersion of light when he was on vacation at home. He set up a dark room, made a small round hole in the windowboard, and put a prism in front of the small hole. He found that sunlight passing through a small round hole diverges after passing through a prism, forming a rainbow-like spot. Dispersion experiments show that sunlight is not monochromatic light, but composed of light of many colors. Newton called the color band formed by dispersion spectrum, and a new discipline was born-spectroscopy.

Dispersion experiments show that sunlight is made up of light of many colors. Wikimedia Commons nearly a century later, Thomas Melville (Thomas Melvill) from Scotland applied a similar experimental method to the flame reaction. In 1752, Melville added sea salt, alum and other materials to burning alcohol, observed the color of the flame with a prism and found that there was a fixed yellow line in the spectrum. Later, William Talbot (William Talbot) and others found that each chemical element will produce its own unique spectral line in the flame color reaction. Melville's yellow line is actually the characteristic spectral line of sodium, which comes from the impurities containing sodium in the material.

Spectral lines produced by flame reactions [6] Flame reactions can produce spectral lines corresponding to chemical elements, and in 1801, British chemist William William Wollaston discovered mysterious dark lines in the solar spectrum. Wallatson improved Newton's experiment by changing the small round hole through which the sun passed into a slit, which was dispersed to get a narrow band of light. Surprisingly, this light band is not completely continuous, but has seven very narrow dark lines.

Portrait of William Watson Wikimedia Commons according to the wave theory of light, each color of light has a corresponding wavelength (wavelength). The wavelength is like the ID card of light, and a spectral line represents light with a certain wavelength. The spectral lines emitted by the flame color reactions of chemical elements have their own wavelengths, while the dark lines in the solar spectrum mean that some wavelengths of light have disappeared. Unfortunately, Watson thought that these dark lines were just the dividing line of color, and did not go any further.

In fact, there are far more than seven lines missing in the solar spectrum, but more sophisticated instruments are needed to find them. German physicist Joseph von Fraunhofer (Joseph von Fraunhofer) was a genius at making glass and was able to make the most sophisticated optical instruments in the world at that time. He invented the first modern spectrometer, which improved the accuracy of the analysis of the solar spectrum. As a result, he found as many as 574 dark lines in the solar spectrum, and marked the main dark lines with A to K numbers. These dark lines were later known as the Fraunhofer line (Fraunhofer lines).

Fraunhofer Line | Wikimedia Commons Why does the Fraunhofer line appear in the solar spectrum? What is the relationship between the Fraunhofer line and the spectral line of the flame reaction? A pair of German scientists, Gustav Kirchhoff (Gustav Kirchhoff) and Robert Benson (Robert Bunsen), finally uncovered the answers to these questions.

Kirchhoff (left) and Bunsen | Wikimedia Commons1859 year, Kirchhoff and Bunsen analyzed the spectral lines produced by the combustion of a large number of chemical elements and made a surprising discovery-the Fraunhofer lines and the spectral lines of some elements are surprisingly consistent in wavelength, and each Fraunhofer line can find the spectral line of an element. For example, the wavelength of D line of Fraunhofer line is about 589nm, which is similar to that of sodium, and the wavelength of L line is about 382nm, which is similar to that of iron and so on.

Part of the solar spectrum drawn by Kirchhoff | reference [2] Kirchhoff also observed in the experiment that when sunlight passes through sodium vapor, the D line in the solar spectrum becomes darker. Combining theory and experiment, he proposed that chemical elements not only emit specific wavelength light (emission spectrum) in the process of flame color reaction, but also absorb specific wavelength light (absorption spectrum) under some low temperature conditions. More importantly, the spectral lines corresponding to the emission and absorption of the same element have the same wavelength.

The emission and absorption spectra of elements have the same wavelength | reference [8] therefore, Kirchhoff concludes that dark lines appear in the solar spectrum because some wavelengths of light are absorbed by elements in the solar atmosphere. In other words, the dark lines in the solar spectrum represent the absorption spectra of the elements on the sun. For example, the wavelength of an emission spectrum of iron is 382nm, because the wavelength of the emission and absorption spectrum of iron is the same, so when the sunlight passes through the atmosphere, the light with the wavelength of 382nm will be absorbed or "intercepted" by iron in the sun, and finally show a dim L line in the solar spectrum received by the earth.

The dark lines in the solar spectrum represent the absorption spectra of elements on the sun. Reference [9] this is an epoch-making discovery. If the emission spectral wavelength of a chemical element is known, and dark lines of the same wavelength are found in the solar spectrum, it means that the element exists in the sun. (some dark lines may be caused by other factors such as the earth's atmosphere.) it is known that the sun is composed of more than 90 elements such as hydrogen, helium, oxygen, carbon, iron and so on, a large part of which comes from the study of the solar spectrum.

Kirchhoff's discovery has more far-reaching implications. Follow-up experiments show that the spectral lines absorbed by the sun can be found in the spectral lines emitted by the elements on the earth, indicating that the elements of the sun and the earth are unified. Physicists also looked further into the universe and found that dark lines also exist in the spectra of stars and nebulae, which are highly consistent with the emission spectra of elements, further proving the unity of the constituent elements of the entire universe. Since then, celestial spectroscopy (Astronomical spectroscopy), a discipline that specializes in studying the properties of celestial bodies through spectra, was born.

We can also deduce the elemental composition of stars from their spectra. [10] imagine how magnificent and wonderful it is that physicists only need to study a few dim rays of light to know the elemental composition of these celestial bodies when we greet the rising sun at the top of the mountain, or look up at the stars at night and marvel at the grandeur of the celestial bodies. The reason why we can peep into these mysteries lies in the wonderful coincidence between the brilliant colors of the fireworks and the mysterious dark lines of the sun.

Reference:

[1] Rigden, John S. Hydrogen: the essential element. Harvard University Press, 2003.

[2] Hearnshaw, John B. The analysis of starlight: two centuries of astronomical spectroscopy. Cambridge University Press, 2014.

[3] Jackson, Myles W. Spectrum of belief: Joseph von Fraunhofer and the craft of precision optics. Mit Press, 2000.

[4] Wang Shuang. Cosmic Odyssey Walking the Solar system. Tsinghua University Press, 2018.

[5] https://www.usgs.gov/media/images/what-minerals-produce-colors-fireworks

[6] https://scientificgems.wordpress.com/2020/03/25/chemistry-can-be-beautiful-the-classic-flame-test/

[7] https://www.spectroscopyonline.com/view/timeline-atomic-spectroscopy

[8] https://webbtelescope.org/contents/media/images/01F8GF9E8WXYS168WRPPK9YHEY

[9] https://publiclab.org/notes/cfastie/3-2-2013/fraunhofer

[10] https://viewspace.org/interactives/unveiling_invisible_universe/analyzing_light/spectrum_of_the_star_altair

Https://en.wikipedia.org/wiki/Thomas_Melvill

Https://en.wikipedia.org/wiki/Joseph_von_Fraunhofer

Https://en.wikipedia.org/wiki/Gustav_Kirchhoff

Https://en.wikipedia.org/wiki/History_of_spectroscopy

This article is from the official Wechat account: bring Science Home (ID:steamforkids), author: everything.

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