Only about 1 percent of people are truly ambidextrous[1] — able to use both hands with equal dexterity to perform a variety of tasks, like writing. For the rest of us, our hands may be mirror images of each other but they don’t function in completely the same way. The same is true of molecules. Two molecules may be identical in terms of chemical composition, but if they are mirror images of each other they won’t function in exactly the same way.
The concept of chirality addresses the orientation of molecules — whether they’re “left-handed” or “right-handed.” We test the orientation of molecules with a polarimeter, which measures the optical rotation caused when we pass light through a given material. How the light rotates tells us the orientation of a molecule. Alice could have made great use of the device 
when she found herself stuck in Wonderland, frustrated that she couldn’t explain herself because she wasn’t herself. Polarimetry would have helped her determine if she was the “real” Alice or the looking-glass version!
To help you visualize chirality, hold your hands in front of you palm up, side by side. Now turn your hands to bring your palms together. The position of your fingers matches up perfectly, thumb to thumb and pinky to pinky, as if you were holding one hand up to a mirror. Next, with your hands side by side, move the left hand over your right. They don’t line up – the pinky of your left hand is above the thumb of your right hand. That distinction is critical in the world of polymer testing.
Here’s another way to understand chirality. If you were to analyze pure sucrose, which has a known polarimetry, your expected value would be +66.37. However, if someone switched your sample, replacing it with “looking-glass” sucrose, you would discover a value of -66.37, indicating the molecules are oriented in a mirror image. Would it taste as sweet as “real” sucrose? Alice didn’t seem to think so.
Alice’s dilemma was the stuff of children’s entertainment, hardly life and death. But in polymer science chirality can affect how a substance behaves during the manufacturing process and in use. For example, Poly(lactic acid) (PLA) is a type of biodegradable polymer that’s used extensively in medical applications. Lactide, the key component of PLA, is an enantiomer — a molecule that has a mirror-image counterpart. Chemically, L-lactide and D-lactide are the same, but the molecular elements that comprise them are mirror images, reversed along one axis. Polarimetric testing can help us identify different types of PLA and determine whether the resin has been enantiomerically enriched or not, because while they are both PLA, they have vastly different properties and behave in very different ways depending on the degree of enantiomeric enrichment.
Polarimetry testing can help us identify a molecule and predict how it will behave based on its orientation. It can also aid us in discovering if a client has accidentally caused racemization to occur in a sample. Additionally, the test can help companies ensure their materials are conforming to specifications.
Poor Alice only had the Mad Hatter and a handful of bizarre characters to try to make sense of the looking-glass world. Fortunately, a polarimeter ensures we can do much better in the world of polymer science!
[1] American Psychological Association, https://www.apa.org/monitor/2009/01/brain.aspx