According to Wikipedia, optical activity was first observed in 1811 in quartz. Louis Pasteur's contribution is also mentioned there and in other sources as a major advancement in establishing optical activity as a distinct characteristic of a compound, specifically as a differentiating factor between two enantiomers - a difference that became a standard descriptive characteristic.

My question is how did they even think of that? Why would Pasteur, Arago, Herschel, and other chemists would think of testing the effect of polarized light on chemical compounds? (vice versa actually, chemical compounds affect polarized light, but I assume they did not know that prior to that discovery...). Was it a standard lab procedure at the time?

Testing and determining physical characteristics (such as melting point, boiling point, etc.) seem reasonable and pretty straightforward, but I am wondering how did polarized light entered the chemistry lab as an identifying procedure.

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    $\begingroup$ Iceland spar was known for ages. Once you are used to the idea of polarized light, it is kind of natural to test it on other transparent minerals. Once you've discovered an interesting effect on some minerals, why not try it on something else? $\endgroup$ Feb 14, 2017 at 11:35
  • $\begingroup$ I would absolutely believe it started with Iceland spar. Scientists of the time would have started with why does it do that? $\endgroup$
    – MaxW
    Feb 14, 2017 at 15:05
  • $\begingroup$ Some minerals create polarised light so you don't need to have the idea first, just observe the properties of the minerals and try to figure out what is going on. $\endgroup$
    – matt_black
    Feb 14, 2017 at 23:18

1 Answer 1


It wasn’t a standard lab procedure to test the effects of polarised light on a certain compound/solution/whatnot. However, back in the day it was standard lab procedure to re-run a certain experiment with practically everything your lab had: water, acids, solvents, table salt, cat hair, human hair, dust, fabric, … You would note if you observed an interesting effect and, where applicable, if that interesting effect would only occur in some cases.

So once you are aware that something like Iceland spar produced interesting phenomena (even though you might not be able to explain these phenomena just yet) you would start trying out all sorts of things to test what this interesting optical phenomenon is capable of doing. By chance, you would run across something that you would be able to observe as cancelling of light, even though all components of the system would let light through — and then you would attempt at understanding what you just observed.

They say science is merely grown men playing around for a reason.

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    $\begingroup$ "They say science is merely grown men playing around for a reason." Totally agree. +1 after 9 hours (used up all 40 votes) $\endgroup$ Apr 21, 2017 at 15:55

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