Why Some of the Rainbow is Missing

- Hello, smart people, this is Joe. In the early 1800s, a German physicist named Joseph von Fraunhofer noticed

some

thing strange. He was looking at sunlight as it passed through a prism and spread over a wall when he realized that part of the

rainbow

was

missing

. Throughout the entire spectrum from red to violet there were dark lines where there should be colors. Fraunhofer couldn't explain what he didn't see, but he eventually cataloged more than 600

missing

pieces of the

rainbow

,

some

dark and others faint. They looked a lot like a barcode, and in some ways, that's exactly what they were.

Deciphering all these little gaps in the rainbow would reveal a hidden story that would eventually allow scientists to unlock many secrets of the universe. This is how they did it and how you can do it too. (brilliant music) The scientists who cracked the Fraunhofer code were named Gustav Kirchhoff and Robert Bunsen. Yes, that Bunsen. And as expected, they were burning things. Kirchhoff and Bunsen were fascinated by how different elements glow different colors when you put them in a flame, such as if you put some table salt in there you will get a bright yellow flame. Try some calcium, it's orange now.

Potassium is kind of pink. Now, to study the light from these flames more precisely, they came up with an instrument called a spectroscope. His spectroscope channeled light from a flame into a prism where it was split into all of its individual wavelengths, known as a spectrum. Then there was another tube they could look through to measure the result. When they looked closer at the colorful chemical flames with their spectroscope, they saw narrow bands of light at specific wavelengths, and no two elements produced the same pattern of bands. This color pattern seemed to be a kind of fingerprint of each element.

For example, sodium has a distinctive yellow line, lithium creates this bright red line, while strontium is red and more, and we can see calcium's strong lines in green and red, although it shines many weaker ones that we can. I do not see. Then, in 1859, these two scientists made the discovery that cracked the Fraunhofer code. Kirchhoff and Bunsen knew about the missing pieces of Fraunhofer's rainbow, and one day after sprinkling some common table salt on Brunson's burner, they realized that the spectral lines radiating from the sodium-filled flame were exactly in the same place as two of Fraunhofer's missing lines.

These lines had to be related, but how? By examining sunlight and a sodium flame at the same time, Kirchhoff and Bunsen were able to show that the bright lines emitted by burning sodium fit like puzzle pieces with certain dark lines missing from Fraunhofer's rainbow. They realized that the elements had a special property. If you heat them, they release light at specific frequencies, but when faced with a full spectrum of light they absorb those same frequencies. He didn't know exactly why this happened, but he concluded that the black Fraunhofer lines were caused by elements in the sun absorbing specific wavelengths.

And together Kirchhoff and Bunsen showed that the spectral lines of many individual elements coincided with missing lines in the spectrum of the sun. By decoding these lines, it would be possible to identify all the elements contained in the Sun without even taking a sample from a hot nuclear ball of gas that is 150 million kilometers away, which would be difficult. Correction, impossible. In the mid-19th century, no one knew why certain elements emitted and absorbed these specific wavelengths. But today we know that the unique spectral signature of each element is directly related to its atomic structure. Each atom has a nucleus surrounded by a certain number of electrons that orbit around it at different energy levels, but most of the time these electrons are at their lowest possible energy level, which is known as the ground state, but they add energy, say by heating things, and some electrons will start to jump to higher energy states.

However, these higher energy states are unstable, so the electrons quickly begin to jump downwards and each time one returns to the ground state, it releases a photon of a very specific wavelength, carrying exactly the same amount of energy it took to lift the electron in the first place. Electrons from different elements reside in unique energy level structures, so you can tell which element you are looking at simply based on the colors it emits when heated. For example, if you heat table salt, you will always see the same two lines in exactly the same part of the spectrum, as certain electrons in sodium absorb energy and then return to their ground state, emitting light.

This is called the emission spectrum. On the other hand, if sodium is in the path of a light source, the electrons in the sodium atoms will absorb whatever wavelength has the exact amount of energy they need to jump to that higher energy level, eliminating those wavelengths. cool. of the spectrum and leaving a shadowed band in its place. Through a spectroscope you see what is called the absorption spectrum. This spectrum subtraction is what happens when sunlight interacts with elements of the solar atmosphere and also with the Earth's atmosphere. As sunlight hits the different atoms in its path, lifting the electrons to higher energy levels, they absorb part of the spectrum, creating the missing rainbow that Fraunhofer saw.

And when you see the sun shining on a normal day, you can't tell that electrons have been stealing bits of light, but if you look closer at that rainbow, you can see that something is missing. But instead of continuing to talk about all this, it would be easier to show him. To make the absorption and emission images you've seen so far, I made this little DIY spectroscope that basically works like Bunsen and Kirchhoff did, except mine, you can illuminate it with a digital camera and it's made from stuff I ordered on Amazon. . They didn't have that.

So here I have an opening for light to come in, passing through a small slit that will help everything look sharper. Then, instead of a prism, I'm using something called diffraction grading which uses small grooves to split the light into all of its individual wavelengths. Just place a camera here and you can see it too. Okay, so I want to see those lines in the sun. By the way, if you do this on your own, never point your spectroscope or your eyes directly at the sun. I'm going to point mine at this white thing that the sunlight is bouncing off.

It is very bright. There's my rainbow and you can see there are dark lines running through it. This is what Fraunhofer saw more than 200 years ago. And thanks to Kirchhoff and Bunsen, we can now decode these lines. These really dark lines that you see are hydrogen lines. The sun is mostly hydrogen, so we have a lot of absorption there, and here's the sodium, the dark line in yellow. In reality they are two lines very close together but we do not have enough resolution to distinguish them. Anyway, there are a lot of lines here, and if we were to look at this through a higher grade spectroscope, we would actually see hundreds and hundreds of them.

They are not all from different elements, most of them are from different energy levels in just a few elements. If we sat down and decoded all these lines, we could decipher every element found in the sun. What more do you want to see? Many light sources around us reveal secrets through the spectroscope. Fluorescent bulbs appear white but reveal specific emission lines rather than a continuous spectrum. Sodium vapor streetlights emit the characteristic sodium line. And neon signs emit the particular emission lines of this noble gas. The amazing thing about spectroscopy is that we can use it to observe things around us or throughout the galaxy.

Well, not with our spectroscope, we need a better one, but in principle it is true. Now that we know the fingerprints of different elements and compounds, physics basically becomes cryology. Based on a pattern of lines, we can decode what things are made of throughout the universe, and in some cases, that cracking of the code uncovers some pretty mind-blowing truths. For example, back in 1912, the American astronomer Vesto Slipher was studying the spectrum of a small fuzzy spot in the sky that he called the Andromeda Nebula. It was an absorption spectrum and had many of the same dark lines that Fraunhofer had seen in the spectrum of our sun.

But the strange thing is that they were not in the right place. They were all shifted towards the blue end of the spectrum. The only explanation Slipher could come up with was that whatever he was looking at had to be moving, because if it were moving towards the Earth while radiating the light, the wavelengths would clump together and appear bluer than they actually were. , like a kind of Doppler Effect. Based on how far the lines moved, Slipher could even tell how fast Andromeda was moving toward us, and the response was ridiculously fast, something like 300 kilometers per second.

Now, after that, Slipher directed his spectroscope at a bunch of other blurry spots in the sky. He still didn't know what they were, but there was also something strange about them. Compared to our sun, all of its spectral lines were shifted toward the red end of the spectrum, which seemed to suggest that there were a lot of fuzzy objects in space moving away from Earth. Slipher didn't realize it at the time, but this was our first hint that the universe is expanding. Those fuzzy dots he called nebulae were actually distant galaxies. About a decade after Slipher, Edwin Hubble realized that the farther away a galaxy was, the faster it was moving away from us, and concluded that the only way that would make sense was if the entire universe was expanding.

The key to this discovery was encoded in those same missing lines that Fraunhofer detected in the solar spectrum a hundred years earlier. Today, this lost rainbow code even helps us in our search for life beyond Earth. When exoplanets cross in front of their stars, we can observe starlight filtering through the exoplanet's atmosphere. New dark lines will appear in the star's spectrum as the planet passes in front and correspond to elements of the chemical compounds in the planet's atmosphere. Some of them could have signs of life. Therefore, finding an unlikely balance of chemicals in an exoplanet's atmospheric spectral signature could be our first clue that something lives there.

The James Webb Space Telescope is already observing the spectra of planetary atmospheres, and who knows what it may one day discover? So, by decoding bits of information missing from sunlight, we have discovered the composition of objects across the universe in places we can never visit or touch. We have discovered strange and mind-blowing truths about the fundamental nature of our universe. We may soon be able to search for life on planets light years away. This is all because some electrons slide off small pieces of the rainbow. Stay curious. I love the fact that something we see all the time like a rainbow and think we know everything about it, still holds secrets like all those missing lines.

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