Rasmus Bartholin

Transcription

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...