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Jury-rigging an electronic eye for a polarimeter.

  1. Jun 19, 2008 #1
    Been too busy with school to post here in a while, so of course I come back when I have a question about something!

    For some background, I'm working on a small study that, in part, is to characterize the optical activity of some biodiesel samples, one made from animal fat, the other from soy beans. The polarimeter I'm using to do it is an ancient, yet very nice, instrument. It's dated to 1934 by the manual that was enclosed in its box, so I'm amazed the lenses are so well preserved.

    Anyway, the first biodiesel sample I was testing looked to have a relatively wide specific rotation range, so the professor I work under had the idea of trying to take measurements using a light sensor so we could graph the results and analyze them statistically.

    It was easy enough to just remove the end focusing lens from the polarimeter and get the light probe into the tube to take some quick readings just to see if it would work, which ended with a really noisy looking dataset, which is why I came here.

    I'm wondering if this is even really possible based on how polarimeters work (I'm a chemist, so this is starting to get outside what I know). So, basically, when you look down the polarimeter you see either a circle with a vertical black bar through it and light glowing on both sides, or a vertical bar of light with the rest of the circle black.

    So, the core of my question is; when you put light through two polarizing filters as such as in this machine, when you rotate one of the filters does the total light coming through the two brighten and darken, or do the bright parts darken and vice versa so there's zero net change in the amount of light coming through?

    If it matters, the sensor I'm using to collect the data is a Vernier LabPro box with this sensor attached;


    And the light source is an ancient sodium arc lamp that GE stopped making bulbs for in the 70s.... The light seems fairly bright and stable, despite its age, and we get good reproducible results when reading the machine by eye with various sugar solutions we know the specific rotation of.

    So it's mostly that I'm wondering whether or not a light sensor that detects changes in brightness would actually work to measure the output of a polarimeter, particularly one that was not intended to ever be read by an electronic instrument.
  2. jcsd
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