- Did spectroscopy, the study of how objects
emit and absorb light,
really revolutionize the sciences and change our world?
In a word, yes.
In chemistry, spectroscopy was used to find new elements.
And infrared spectroscopy is still used
in chemical analysis to this very day.
In astronomy, spectroscopy allowed us to figure out
what the sun and stars are composed of
and it's the most powerful tool in the astronomers toolbox.
And physics?
Physics is where spectroscopy had the most input
for spectroscopy directly led
to the development of quantum mechanics.
And all of this began when a shy small physicist
named Gustav Kirchhoff suggested
that his friend, a large absentminded chemist
named Robert Bunsen use a prism.
Ready for one of my favorite stories. Let's go.
♪ Electricity ♪
- Kirchhoff and Bunsen met because of a revolution.
Several revolutions actually.
See in 1848 young Gustav Kirchhoff won a scholarship
to travel from Prussia to France
for his astonishing work that he'd done as an undergrad
on how complicated circuits follow Ohm's Law
that is still causing students in engineering and physics
to butcher the spelling of his name today.
However, the summer of 1848 was the beginning of quote,
"The most widespread revolutionary wave
in European history."
Therefore Kirchhoff only made it as far as Berlin.
Undeterred, he talked his way into a temporary position
at the University of Berlin
and then in a position at the University of Breslau,
where he became close friends
with a visiting chemist named Robert Bunsen.
Bunsen was a renowned chemist,
but also an eccentric character.
So famous for his amusing quirks that his friends wrote
an entire book about his humorous stories.
For example, Bunsen was a lifelong bachelor,
but according to legend, he once asked a woman for hand
in marriage and had been accepted.
However, after that, he got so involved in his research,
he forgot to talk to his fiancee for several weeks.
He also forgot whether he had proposed or not.
Undeterred, he just went to the young lady
and proposed again.
Insulted, she changed her mind
and promptly showed him the door.
Anyway in 1852, Bunsen got a chemistry position
at the university of Heidelberg and used his influence
to get Kirchhoff a physics position there too.
A few years later, Bunsen heard that the town
was to be supplied with gas, to light the streetlamps
at light and homes at night.
Bunsen then worked out a deal with the gas company
to supply his laboratory with gas during the day.
Bunsen then realized that the gas burners available
were unsteady and did not produce a lot of heat.
He then set out to make a better one.
And by 1857 published the description of a Bunsen burner,
a device still in use in chemistry laboratories
throughout the world.
However, Bunsen would never think of patenting his device
writing once that "working as beautiful and rewarding,
but acquisition of wealth for its own sake is disgusting."
Bunsen thus had a very good heat source to play with
and decided as he was a chemist
to systematically study the light produced
by different chemical salts when they burned.
Bunsen tried to qualify the colors
by studying them through colored filters,
which didn't work very well.
One day in the summer of 1859, he complained to Kirchhoff
who wondered why he didn't just use a prism.
Physicists had known since Newton's time,
that prisms bend different colors by different amounts
and could make a rainbow.
However, when they looked at heated low density gases
with a prism, they didn't see a rainbow.
In fact, the gases would make sharp bars
of specific frequencies called spectral lines.
This is not to say the Bunsen and Kirchhoff
were the first to study spectral lines with a prism.
They weren't.
What made this different is that Kirchhoff was suggesting
that they use a movable arm so they could systematically
study the colors produced by different elements.
They soon found that each element
had its own unique optical fingerprint.
What is going on?
Well, the electrons in the gas are bound to the nucleus
and can only exist at certain energy levels.
When they're heated,
the electrons will jump up energy levels.
Then when they fall back down,
they release a photon of light whose energy and color
depends on the change of energy levels.
Different elements have different energy levels.
So they produce different bands of light.
Bunsen and Kirchhoff built their first spectroscope
out of Bunsen's old cigar box, some telescope parts,
a prism, and of course, a Bunsen burner.
Despite being crude,
this was a surprisingly sensitive device.
Bunsen noted that quote
"Chemistry produces no single reaction,
which in the remotest degree can compare in sensitiveness."
They found, for example, they could see sodium
when as little as one out of 20 millionth of it
was suspended in smoke.
They also found that they could burn compounds
and from their fingerprints
determine what they were composed of.
Bunsen and Kirchhoff burned some compounds and found some
fingerprints that didn't correspond to any known chemical.
And in this way, discovered two unknown elements,
rubidium and cesium.
That's not all, in October Kirchhoff was playing
with a fingerprint of sodium and as a lark
decided to add the light from a bright lamp.
Kirchhoff knew that his lamp, like most heated liquids
and solids and plasma gas,
would produce basically continuous spectrum or rainbow.
Therefore with both a lamp and heated sodium,
he expected to see a rainbow with two extra
bright yellow lines from the sodium.
To Kirchhoff's shock he got a rainbow
with two black bands in the yellow, exactly at the spots
where the bright lines were supposed to show.
Excitedly this rainbow with black bands in it
looked like sunlight.
See sunlight is not actually continuous rainbow,
but if you study it with a really good prism
and a microscope, you will see little dark bands in it.
The fact that sunlight has dark bars was first discovered
in 1802, but was made famous 12 years later
by a German glass maker named Joseph Fraunhofer
who counted over 570 lines in sunlight.
Kirchhoff therefore realized that he was recreating
the Fraunhofer lines of sunlight with lamplight and sodium.
It seemed immediately clear to Kirchhoff
that the shadows in the yellow part of sunlight
must be due to the sodium gas on the sun.
Let me explain in detail because this is both
really amazing and a little bit complicated.
The sun is a hot plasma where the elements are so dense
and hot that the electrons are ripped free from their atoms
and are free to move.
Therefore, the electrons in the sun
produce a continuous spectrum of light.
The atmosphere of the sun, however,
has spots cool enough for the elements to be in gas form
instead of plasma form, although they are still ridiculously
hot, hot enough that even the metals are in gas form.
As these gases are cooler than the sun,
they absorb more radiation than they produce.
Therefore, the gas elements in the atmosphere of the sun
will leave the shadows
of their optical fingerprints in sunlight.
By studying the optical fingerprints on Earth
and comparing it to the optical shadows from the sunlight
Kirchhoff had found a way to determine
what the sun was composed of.
Soon Kirchhoff and Bunsen found many different elements
in the sun, but no helium as helium was actually discovered
because of the shadows in the sunlight,
which is why helium is named after Helios the Greek sun god.
Kirchhoff and Bunsen even found
trace amounts of gold in the sun. It's true!
About 60th of a billionth of a percent of the sun
is made of gold gas, but as the sun is so large,
there's about as much gold in the sun
as water in the oceans of the earth.
Kirchhoff liked to tell the story of how his banker
was unimpressed about finding gold in the sun
as he couldn't bring it to Earth and put it in his bank.
Soon after, Kirchhoff won a prize for this work
and told his banker, "Look here, I have succeeded at last
in fetching some gold from the sun."
But wait, we aren't done yet.
In 1860 Kirchhoff wrote a theoretical physics paper
about the emission and absorption of light.
In this paper Kirchhoff imagined a perfect object
that would completely absorb all incident rays,
which Kirchhoff called perfectly black
or more briefly black bodies.
If it absorbed all incident light, then any light
that emitted from a black body would be
from the substance itself being heated
and Kirchhoff theoretically predicted the amount of light
you get only depends on the temperature of the substance
and the frequency you're looking at.
Kirchhoff didn't have a theoretical form for this equation,
but he felt that quote "To determine this function
is a matter of the greatest importance."
Although he was sure of success since the form
of the function in question is no doubt simple.
The equation turned out to be pretty simple,
but it was very difficult to create the experiment
and find a good theoretical logic behind it.
In fact, it took 40 years
to solve the black body radiation puzzle.
In 1900, a man named Max Planck wrote a paper
deriving the black body equation.
Whereas he put it quote, "The most essential point
of the entire calculation is that energy is composed
of very definite number of equal finite packages."
Five years later, Einstein called these energy packets
Quanta of light.
In other words, in 1900 Max Planck
started the quantum mechanics revolution
because of black body radiation.
And that story is next time on the Lightening Tamers.
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Speaking of special videos, I am making a special video
with a lot of the stories about Bunsen
that I couldn't include in this video,
as well as a story about Fraunhofer,
the guy who discovered that visible light
has those dark lines in it, why he did that
and why he could only do that because
a building collapsed around him when he was a teenager.
Also I'm including a little bit about a husband and wife
team who were pioneers in using spectroscopy
with astronomy and with photography.
They're especially interesting because William Huggins
was quite willing to let his wife work with him
but was simultaneously working against women
getting acknowledgements in science.
it's a very interesting story or a very annoying story,
depending on your point of view.
By the way, if you want to see the video,
but you don't have the money to be a Patreon.
I understand. I've a link to an email,
shoot me an email and I'll send you a copy of the video.
I'm a sucker.
Okay. Have a nice day.