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Steve Mould did a nice video a few months back called "Why Sugar Always Twists Light To The Right" (link is timestamped at the critical part).

Steve Mould

However his explanation was not complete.

He explains that molecules that have mirror symmetry, ultimately cancel each other out because any polarized light that gets warped by one molecule, gets de-warped by the next "mirrored" molecule.

However, he continues, chiral molecules (which are like hands, in that left and right cannot be "stacked"), don't do this because they don't have mirrored molecules, so any effect on polarized light passing through never gets "undone". It only gets "twisted" in one direction.

Here's what remains unexplained: Even two left hands CAN be stacked in such a way that they WILL cancel each other out. Yes perhaps they will not be oriented in a "high five pose", but in a floating stew why can't they be lying on top of eachother in such a way that they completely cover eachother and undo eachother's effect?

CodyBugstein
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  • Sugar is a generic name. What Mould probably meant was that sucrose always twists (plane polarized) light to the right. (+)-Dextrose (D-glucose) is dextrorotatory and (-)-levulose (D-fructose) is levorotatory. Both of them are sugars. – user55119 Sep 10 '20 at 22:54
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    A right-handed screw, turned upside down, is still right-handed. – Karsten Sep 11 '20 at 01:53
  • @Mithoron A bit but I still don't get why there aren't "upside down" molecules in the stew as well – CodyBugstein Sep 11 '20 at 02:07
  • This question is basically a duplicate of that linked in the comment by @Mithoron – Alchimista Sep 11 '20 at 08:23
  • Take the spring out of a ball pen, and have a good look. It's chiral, and the twist cannot be undone. A flat molecule (a flat hand, neglecting the different palm and back side) is NOT chiral. – Karl Sep 11 '20 at 10:18
  • Two left hands don't "cancel each other out" they reinforce each other. If they "completely cover each other" they both have the same effect. – matt_black Sep 11 '20 at 12:49
  • I comment again because the doubt of OP seems to rises because s/he is missing the mechanism of optical rotation. Thus this discussion togheter with the above should fix the issue: https://physics.stackexchange.com/questions/15503/how-do-optically-active-compounds-rotate-plane-polarized-light – Alchimista Sep 12 '20 at 03:14
  • The key point is that you need the high-five orientation for the two effects to cancel each other out. – Jan Sep 17 '20 at 07:56

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