- 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.
I just love the fact
that something that we see all the time like a rainbow
and we think we know everything about it,
it still holds secrets like all of those missing lines.
And we love to bring you stories like that about science
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