- Hey, smart people, Joe here.

In the early 1800 a German physicist named Joseph von Fraunhofer noticed something strange.

He was looking at sunlight as it passed through a prism and spread out on a wall when he realized part of the rainbow was missing.

All throughout the spectrum from red to violet there were dark lines where there should have been colors.

Fraunhofer couldn't explain what he didn't see but eventually he cataloged over 600 pieces missing from the rainbow, some were dark, some were faint.

They looked a lot like a barcode and in a way 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.

(bright music) The scientists who cracked Fraunhofer's code were named Gustav Kirchhoff and Robert Bunsen.

Yes, that Bunsen.

And just as you expect, they were burning stuff.

Kirchhoff and Bunsen were fascinated by how different elements glow different colors when you put them in a flame, like stick some table salt in there and you'll get a bright yellow flame.

Try some calcium, now it's orange.

Potassium is kind of pinkish.

Now to study the light from these flames more precisely they came up with an instrument called a spectroscope.

Their spectroscope channeled light from a flame toward a prism where it got split into all of its individual wavelengths, what's 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 pattern of colors seemed to be a kind of fingerprint for each element.

For instance, sodium has a distinct yellow line, lithium creates this bright red line, while strontium is red and more, and we're able to see calcium's strong lines in green and red, though it shines many other weaker ones that we can't see.

Then in 1859, these two scientists made the discovery that cracked Fraunhofer's code.

Kirchhoff and Bunsen knew about Fraunhofer's missing pieces of the rainbow, and one day after sprinkling some common table salt on Brunson's burner, they realized that the spectral lines beaming from the sodium filled flame were in the exact same spot 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 elements had a special property.

If you heat them, they release light at specific frequencies but in front of a full spectrum of light they absorbed those same frequencies.

He didn't know exactly why this happened but concluded that Fraunhofer's black lines were caused by elements in the sun absorbing specific wavelengths.

And together Kirchhoff and Bunsen went on to show that many individual elements' spectral lines matched those missing lines in the sun's spectrum.

By decoding these lines, it would be possible to identify all of the elements the sun contains without ever taking a sample of a hot nuclear ball of gas that's 150 million kilometers away, which would be difficult.

Correction, impossible.

Back in the mid 1800s, no one knew why particular elements gave off and absorbed these specific wavelengths.

But today we know that each element's unique spectral fingerprint is directly tied to its atomic structure.

Every atom has a nucleus surrounded by some number of electrons orbiting around in different energy levels but most of the time these electrons sit in their lowest possible energy level, what's known as the ground state, but add energy, say by heating things up, and some electrons will start jumping up to higher energy states.

These higher energy states are unstable though so electrons quickly start jumping back down and each time one returns to the ground state it releases a photon of a very specific wavelength, carrying the exact same amount of energy that it took to bump the electron up in the first place.

The electrons of different elements reside in unique structures of energy levels so you can tell what element you're looking at just based on the colors it emits when it's heated.

For example, if you heat table salt you'll always see the same two lines in the exact same part of the spectrum as certain electrons in sodium absorb energy and then return to their ground state, giving off light.

This is called an emission spectrum.

On the other hand, if the sodium is in the path of a light source, electrons in the sodium atoms will absorb any wavelengths that have the exact amount of energy that they need in order to jump up to that higher energy level, removing those wavelengths from the spectrum and leaving a shadowed stripe in their place.

Through a spectroscope, you see what's called an absorption spectrum.

This spectrum subtraction is what happens as sunlight interacts with the elements in the sun's atmosphere and in earth's atmosphere too.

As sunlight hits all the different atoms in its path, bumping electrons up 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 an ordinary day you can't tell that electrons have been stealing bits of light but if you take a closer look at that rainbow just right, you can see that some of it is missing.

But rather than keep just talking about all this, it would be easier to actually show you.

To make the absorption and emission images that you've seen so far, I made this little DIY spectroscope that basically works like Bunsen and Kirchhoff's did, except mine, you can shine a digital camera into it and it's made of stuff I ordered on Amazon.

They didn't have that.

So here I have an opening for the light to go in, passing through a tiny little slit which will help make everything look sharper.

Then instead of a prism, I'm using something called a diffraction grading which uses tiny little grooves to split light into all of its individual wavelengths.

Just put a camera right here and you can see it too.

Okay, so I wanna check out those lines in the sun.

By the way, if you do this on your own, never ever point your spectroscope or your eyes directly at the sun.

I'm just gonna point mine at this white thing that has sunlight bouncing off of it.

It's very bright.

There is my rainbow and you can see there are dark lines running through it.

This is what Fraunhofer saw over 200 years ago.

And thanks to Kirchhoff and Bunsen, we can actually decode these lines now.

So these really dark ones you see, those are hydrogen lines.

The sun is mostly hydrogen so we have a lot of absorption there, and here is sodium, the dark line in the yellow.

It's actually two lines really close together but we don't have enough resolution to make out both of them.

Anyway, there are a lot of lines here, and if we were to look at this through a higher grade spectroscope we'd actually see hundreds and hundreds of them.

They're not all from different elements, most of them are from different energy levels in just a few elements.

If we were to sit and decode all these lines we could actually figure out every single element that's in the sun.

What else you wanna look at?

Many light sources around us reveal secrets through the spectroscope.

Fluorescent light bulbs appear white but reveal specific emission lines instead of a continuous spectrum.

Sodium vapor street lamps emit the characteristic sodium line.

And neon signs give off the particular emission lines of this noble gas.

The amazing thing about spectroscopy is that we can use it to look at stuff that's right around us or across the galaxy.

Well, not with our spectroscope, we need a better one, but in principle, it's true.

Now that we know the fingerprint of different elements and compounds, physics basically becomes cryology.

Based on a pattern of lines, we can decode what stuff is made of all over the universe, and in some cases, that code breaking uncovers some pretty mind blowing truths.

For instance, back in 1912, the American astronomer Vesto Slipher was studying the spectrum of a fuzzy little spot in the sky that he called the Andromeda Nebula.

It was an absorption spectrum, and it had a bunch of the same dark lines Fraunhofer had seen in our sun's spectrum.

But the weird thing was they weren't quite in the right place.

They were all shifted toward 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 toward earth as it radiated the light, the wavelengths would bunch up and appear bluer than they actually were, like a kind of Doppler effect.

Based on how much the lines were shifted, Slipher could even tell how fast Andromeda was moving toward us, and the answer was ridiculously fast, something like 300 kilometers a second.

Now, after that, Slipher turned his spectroscope to a bunch of other fuzzy spots in the sky.

He still didn't know what they were but there was something weird about them too.

Compared to our sun, their spectral lines were all shifted to the red end of the spectrum, which seemed to suggest there were a bunch of fuzzy objects in space hurdling away from earth.

Slipher didn't realize it at the time but this was our first hint that the universe is expanding.

Those fuzzy spots that he was calling Nebulas were actually far off 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 he concluded that the only way that made sense was if the entire universe was expanding.

The key to this discovery was encoded in those same missing lines Fraunhofer spotted in the sun's spectrum a hundred years earlier.

Today, this missing rainbow code is even helping us in our search for life beyond earth.

As exoplanets cross in front of their stars we can look at the starlight that filters through the exoplanet's atmosphere.

New dark lines will appear in the star's spectrum as the planet crosses in front and they correspond to the elements in chemical compounds in the planet's atmosphere.

Some of those could hold signs of life.

So finding an unlikely balance of chemicals in an exoplanet's atmospheric spectral fingerprint could be our first clue that something's living there.

The James Webb Space Telescope is already looking at the spectra of planetary atmospheres and who knows what it may one day discover?

So by decoding bits of information missing from sunlight we've figured out the makeup of objects all over the universe in places we will never be able to visit or touch.

We've discovered bizarre mind-boggling truths about the fundamental nature of our universe.

We may soon be able to search for life on planets that are light years away.

All of this is thanks to some electrons swiping little bits of the rainbow.

Stay curious.