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I recently saw this awesome video by Steve Mould where he explained that a sugar solution in water will turn polarized light in the clockwise direction.

The explanation basically boils down to sugar molecules (glucose) having a handedness (they are chiral) and that linearly polarized light can be thought of as a superposition of circular polarized light in opposite directions which experience a different refractive index when interacting with the sugar solution.

Now to my question; If I want to replicate this experiment at home, will regular table sugar work, or do I need pure glucose, and if that is the case where can I get it?

Many thanks!

Edit 1: I will get back with the results I get from using table sugar when I have performed the experiment.

Edit 2: I did the experiment using half water half sugar, basically simple syrup, and the result was excellent. The optical rotation was very apparent.

Turbotanten
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  • Just beware of the table sugar being used by Jules Verne Durand & his gang :-) – Carl Witthoft Aug 25 '20 at 11:24
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    Re, "where can I get it?" Have you looked? When I typed "pure glucose" into Google, I got some promising results. – Solomon Slow Aug 25 '20 at 13:21
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    https://en.wikipedia.org/wiki/Inverted_sugar_syrup - it is called inverted because the mixture (glucose and fructose) rotate the polarization in the opposite direction compared to the sucrose solution one starts with. – fraxinus Aug 25 '20 at 19:19

3 Answers3

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Chemically, table sugar is sucrose, whose molecule is basically a unit of glucose and a unit of fructose connected together. To know the expected amount of rotation of polarization for a given substance, see the table of specific rotations.

In particular, for D-glucose specific rotation is $+52.7°\,\mathrm{dm}^{-1}\,\mathrm{cm}^3\,\mathrm{g}^{-1}$, while for D-sucrose it's $+66.37°\,\mathrm{dm}^{-1}\,\mathrm{cm}^3\,\mathrm{g}^{-1}$, which is actually even larger than that of D-glucose. So yes, you should be able to succeed with the experiment using table sugar instead of glucose.

Ruslan
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  • Thanks! My initial guess was that since sucrose is made up of glucose and fructose, and fructose rotates polarized light in the opposite direction of glucose that the net effect of the twisting of polarized light for sucrose would be zero. – Turbotanten Aug 25 '20 at 09:38
  • @Turbotanten well, they rotate by different amounts, so even if the net effect were a simple addition, you'd still get something nonzero. – Ruslan Aug 25 '20 at 09:43
  • Yes that's what I figured! :) – Turbotanten Aug 25 '20 at 09:43
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    @Turbotanten optical rotation has a tendency to be nonadditive once new chemical interactions are involved. This has led a lot of difficulty in computational prediction of OR for solutions as you can't simply determine the OR of the molecule and the OR of the surrounding (chiral) solvent cavity separately. – Tyberius Aug 25 '20 at 21:31
  • dm^-1? what is this unit? – Stack Tracer Aug 27 '20 at 04:22
  • @StackTracer reciprocal decimeter – Ruslan Aug 27 '20 at 06:17
  • @Ruslan I thought it might be that, but I must admit that I'm confused as to why you'd ever want to have cm^3 and dm^-1 next to each other and not just reduce the unit to cm^2. – Stack Tracer Aug 28 '20 at 22:11
  • @StackTracer well, that unit was used in Wikipedia (see the link in the answer), which references CRC Handbook of Chemistry and Physics, and the tables there use these units. I suppose decimeter is just closer to the actual experimental scale on which the measurements are usually done. After all, it's easier to measure ~60° of rotation than ~6°, especially for such a quantity as polarization, whose measurements usually use Malus' law that contains quite a smooth function of angle. Also, compare this with spectral irradiance, where nm is normally not flattened to m, despite the m² term. – Ruslan Aug 28 '20 at 22:18
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I have successfully done this demonstration in my classes using table sugar. I place a polarizer on an old-fashioned overhead projector, hold a crossed polarizer above it to block the light, and then insert a beaker of sugar solution between the two. I usually use a solution of 1:1 sugar-to-water (by volume), basically a "simple syrup"; and I use a column depth of about 10 cm or so.

If I remember correctly, the transmitted light has a distinct bluish color when you do this, which I assume is due to the frequency dependence of the specific rotation.

Michael Seifert
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    Also, I'm amused that this question has given me (a theorist) a chance to post an "experimental" answer. – Michael Seifert Aug 25 '20 at 17:32
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    The coloration is an interesting detail I didn't even think to think of :) – Ruslan Aug 25 '20 at 17:32
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    Exciting! I have, over the past year, often found myself lamenting how hard it is to find overhead projectors these days. They are very useful for undergraduate demonstrations... – Philip Aug 25 '20 at 17:48
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    @Philip: I think I've also successfully done this demo using a document projector with a backlight. But I don't know how many document projectors have this feature. – Michael Seifert Aug 25 '20 at 18:57
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Yes, I helped my daughter do this demonstration for her sixth grade science project. She used plain table sugar and a laser pointer as the light source. Having a monochromatic source makes it a little clearer. If you have different color laser pointers it would be interesting to demonstrate wavelength dependence.

Brent Meeker
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