What kind of research did Joseph Fraunhofer do on light?

The measurement of the speed of light is a major technological development, but the most important technology is not the study of the speed of light, but the study of the color of light.

Newton observed the properties of light as it passed through a prism. As he rigged up his experimental setup, he produced a spectrum of light on the screen behind the prism, a rainbow. The so-called "red shift" and "blue shift" are based on the spectral position.

Newton discovered that white light is not pure, and white light is the least pure light. White light can be divided into multiple colors, and multi-colored light can be synthesized into white light.

Joseph Fraunhofer (1787~1826) was a mirror grinder and glassmaker in Munich. He once designed precision grinding machines, he also improved telescopes, and was very familiar with the properties of various glasses and knew how to process them into high-quality optical instruments.

Fraunhofer compared the refraction of light from various glasses, allowing sunlight to pass through a prism made of a single glass, but he found that: because the colors of the spectrum are concentrated in a smaller range, a It is impossible to make precise comparisons in the first place. So Fraunhofer drew up a plan to further expand the spectrum.

As a result, the Fraunhofer Line was born.

The colors of the solar spectrum are not gapless and continuous. What can be seen from the spectrum is that there are irregular narrow spectral line distributions. This is the Fraunhofer line.

Fraunhofer believes that "these spectral lines prove that the components of the decomposed white sunlight are not composed of continuous spectra with different refractive powers, and prove that the light comes from a certain color level, so the dark lines are the gaps in the spectrum, which correspond to the missing light. If this spectrum were produced by sunlight passing through a prism made of the same material each time, these spectral lines would always be in the same part of the spectrum, in the same order and position. Density and lightness are the same. If the materials are different, the quantity, order, and lightness will not change, but the mutual distance between the spectral lines will be different."

People have always believed that the sun and other stars are the same light species, but Fraunhofer discovered that the spectrum of stars is different from the spectrum of the sun.

This triggered an important study, namely spectral analysis. Spectral analysis was a major scientific achievement in the 19th century. Due to spectral analysis, chemists could point out the conditions of tiny elements, and astronomers also began to move towards astrophysics. As for metallurgy, engineering, etc., trace amounts of substances can also be accurately determined to determine quality and accidents.

At that time, people were making use of the relationship between elements, atoms and light, but why could they keep glowing and have different colors? People in the 19th century didn't know that this was the scope of atomic physics.

The "Bunsen burner" in the laboratory today is a technical tool invented by the scientist Bunsen. It is a gas lamp with a sufficient air supply. Due to the sufficient supply of air, this flame has almost no color, and the heat is very high, which is very helpful for observing color.

The German chemist Bunsen (1811~1899) and his colleague Kirchhoff (1824~1887) used this lamp to study the combustion and luminescence of many elements.

They used platinum wire to slowly bring various salts close to the flame, and they could observe the spectrum of the vapors burning on the salts. "The phenomena before us are among the most brilliant optical phenomena ever created by man. Now we see only the spectrum corresponding to burning salt, which appears with maximum brilliance, whereas in previous experiments the largest features of the spectrum were Obscured by the light of alcohol”.

Bunsen and Kirchhoff concluded that metals have their own special flame reaction. In order to further make the metal compound that is not easily soluble show a flame reaction, the two of them also used electric sparks, because the fire provided by electric sparks is very strong.

The spectrum of incandescent solids is continuous. Since the spectrum of an element is independent of the compound in which it is contained, a good way to examine an element is through its flame reaction. In the test, the spectra of the various elements of a compound do not interfere or affect each other. But the main thing is that the validation method provided by Bunsen and Kirchhoff shows great sensitivity. Bunsen described that in one experiment three millionths of a milligram of sodium was sufficient to obtain a clear spectrum.

Using spectral analysis, people soon discovered some chemical elements that had been ignored in research because they only appeared in extremely small amounts. For example, rubidium and cesium were discovered through the color of flames. Later, through spectroscopy, the presence of indium, gallium, and scandium was discovered. The composition of unknown compounds can also be determined by spectral analysis.

Fraunhofer once observed that the two dark lines of the solar spectrum coincide with the bright lines of the sodium spectrum in laboratory experiments. Léon Foucault, Bunsen and Kirchhoff explained it this way: If bright light falls on less bright sodium vapor, then a "reversal of the sodium line" will occur. The position of the bright line in the spectrum is now darker than the rest. The same is true for the spectral lines of other chemical elements, using corresponding experimental methods.

What is the reason?

Glowing gases and vapors absorb the colors they emit. In addition to the light-induced emission spectrum of the luminophore, there is also the absorption spectrum. When light passes through luminescent gases and vapors, it produces an absorption spectrum.

At this time, the absorption spectrum is to some extent the "opposite" of the emission spectrum. The position of the dark line belonging to a certain element in the absorption spectrum is exactly where the bright line of the emission spectrum would be if there was no absorption.

This understanding explains the formation of Fraunhofer lines in the solar spectrum.

Kirchhoff wrote:

"In order to explain the dark lines in the solar spectrum, it must be admitted that the atmosphere of the sun surrounds the luminous body, and the luminous body itself only produces a spectrum without dark lines . The assumption one can make is that the sun is a solid or fluid high-temperature core surrounded by a slightly cooler atmosphere."

Elements in the Sun's atmosphere absorb "its own" light, thus forming dark lines. In fact further measurements and comparisons showed that many elements on Earth are hot vapors in the solar atmosphere. As long as we expand the spectrum of stars and study them, we will find that elements "on Earth" also exist in stars.

In the history of chemistry, there is an element that was discovered for the first time on the sun.

At that time, people already knew how to place and darken telescopes equipped with spectrographs to obtain the spectrum of the hot gas layer surrounding the sun, rather than the spectrum of the sun itself. Therefore, the spectroscope shows not the absorption spectrum, but the emission spectrum. The normally dark Fraunhofer lines appear brighter. British astronomer and physicist Joseph Norman Lockyer observed a bright yellow line here, which belonged to an unknown element. Lockyer hypothesized that the cause was the existence of an unknown element on Earth, which he named helium. It took almost 30 years before helium was discovered on Earth in 1895, and trace amounts of helium were found in certain minerals. New elements were first discovered in the sun and later on the earth, which is compelling evidence that the same elements are also found in celestial bodies.

Since then, spectral analysis has made great achievements in astronomy and astrophysics.

People can infer the temperature of the surface atmosphere from the spectrum of the planet, and from this, the key points of the temperature of the star itself can be obtained.

There is a subtle shift in the spectrum of the light source that can only be measured with the most sophisticated means. The shift depends on the speed of the light source moving towards or away from us. Based on this, we can Use spectral analysis to determine stellar velocities.

The rapid development of photography technology in the 19th century made contributions to spectral analysis.

Currently, spectral analysis has evolved from visible light to invisible light, and can determine the chemical composition of distant planets, proving the ubiquity of chemical elements.