So last week on Tech Laboratories we looked at calcium carbonate, the cause of a
150-year-old
physics problem - or rather a problem that was over 150 years old when it was
finally solved in
1821
That's right, today I'm talking about the crystalline form calcium carbonate
calcite. So as we learned last week calcite is the most stable crystalline form of
calcium carbonate
unit cells consist of two calcium carbonate molecule stacked on top of
each other with the 60-degree twist that looks like this
and then it grows out into a full crystal which gives you the nice crystalline
form that we know as
calcite. In its clearest crystalline form calcite is often called
Icelandic spar because it's found in fairly large quantities in Iceland though most of it
today comes from Mexican. In 1669 a Dutch scientist named Rasmus Bartholin
or
Erasmus Bartholinus, if you wanna pronounce it in the Latin version of his
name, published a book and this book was called
this book was called
Or as we might roughly translated into English
Experiments with Grouped Light Separation of Icelandic Spar: A Marvelous and Unusual
Refraction Unmasked
now that's a title for a book! So basically this is a sixty-page book
where he describes the
image doubling of calcite called birefringence. He describes how one of
the images follows the
ordinary refraction seen with lenses and other materials, but the second
image follows a more extraordinary set of laws and appears to rotate around the
ordinary image as you rotate the crystal.
Bartholinus was probably not the first to see this, but he is the first to write
it down and therefore he's credited with the discovery.
This property of calcite was a problem for physicists as they tried to explain
light - from there on if you wanted to explain how light work you had to
explain birefringence.
In 1690 Christiaan Huygens published his very important work called Traite de la Lumiere
where he explains the wavelet
theory of light, though he never actually calls them waves, any never thought
about them having a frequency or an
amplitude, he is considered to be the father of wave theory in light. The book was based
on work he presented in 1678
What was he doing in the twelve years he was correcting errors
and oh yeah - writing about calcite. so basically Huygens way of explaining
what's going
on with calcite was "maybe instead of circles, they're ovals?"
Huygens also did a couple really important experiments on calcite.
He took
two pieces of calcite, placed one in front of the other and discovered, no, the second
images don't double
again and then second that if he rotated the one crystal by 90 degrees the
ordinary ray became the extraordinary ray and the extraordinary ray became the
ordinary ray in this second crystal. So he didn't really have a mathematical
explanation for what's going on so
yeah. In 1704 Isaac Newton publishes his work on my called
Opticks, where he proposes that light does not travel as a wave as Huygens
suggested but
in corpuscles, or rays as he calls them - what we would call
particles today. his explanation for calcite was that
it was being affected by this sides the
corpuscles... yeah I don't get it either
after Isaac Newton publishes his work very little is added to our
understanding of light for the next 100 years
until 1801 when Thomas Young presents the results of his now famous double
slit experiment
which basically says that light behaves like a wave.
but even though they now had very strong evidence to support light as a wave, very
few people still believed it and no one had any better explanation as to what
was going on with calcite.
In 1808 Frenchmen Etienne-Louis Malus was sitting in his apartment in Paris on
La Rue d'Enfer looking at the Luxembourg palace at sunset he does what
any normal person would do and picks up his calcite crystal and starts looking
through it and playing with it
sometimes I sit at home alone playing with my rocks... he discovers something
really really strange - as he rotates the crystal part of the
brightness of the Crystal changes, something that didn't happen when he
looked at the Sun directly - so the scientist part of him started thinking
What? Malus started doing some experiments with reflecting light
off of surfaces and discovered that at specific
angles the light would become what he called polarized, meaning it was
separated in some way that he could not describe, where when seen through a
calcite crystal it would diminish in brightness. So Malus worked out that
with different angles of rotation between two reflective mediums he could
allow
or prevent the light from being reflected, and that the amount like reflected
by the analyzing surface was related to the cosine of the angle between the two.
We call this Malus' Law.
In essence he was describing what Christian Huygens had already found
out, but Christian Huygens never described it mathematically and never coined the
term polarization, so thanks Malus! This is where the story of calcite and
polarization diverge
In 1828 a scientist named William Nicol cuts calcite and creates the
first transmissive polarizer
light passes through and exits polarized. The Nicol prism became extremely
important in microscopes and all kinds of scientific equipment from there on out
Similar prisons are still used today in high wattage output But that's not
everything for the history of polarization. Join me next time on Tech
Laboratories where we finish the history of polarization and then again when I'm
going to be attempting to cut my own calcite polarizing prisms - which by the way
is
really hard to do! For more weekly science content don't forget to
subscribe to Tech Laboratories, like this video, share with your friends, and keep
videos like this
coming. Until next time, I'm Tech Adams, saying keep thinking, and thanks for
watching.
and that everything else in between was related to the sine law. Sine law? Or Tangent? I should probably look that up...
Some days, I hate math. And by hate
I mean, I absolutely love it, but it's just kind of annoying.
Hello...Malus Law...
Cosine. Ah!
And that the amount of light transmitted was related to the cosine of the angle...