Why do chiral molecules rotate polarized light?

Suppose you have an identical twin, the same dimensions as you (and I assume you are brothers). Now shake hands........Your right hand fits in his right hand, you can also shake left hand to left hand; the handses fit, they overlap (interestingly the twins WOULD have different sets of fingerprints, but this is by the by). But why can't you two shake left hand to right hand or vice versa. Try this with a partner.......Without a compliant twin, can you tell me if it is easy to put your left shoe on your right hoof? Sometimes, even without looking at your shoes your feet can feel the right geometry.......

Now extend the phenomenon to plane-polarized light, which interacts differently with each (homochiral) stereoisomer. You know that light interacts with matter in specific ways and specific frequencies - this is the underlying principle of all spectroscopy. The interaction between plane-polarized light and homochiral enantiomers is one of these........

In principle, if a solution of a homochiral isomer rotates light clockwise by an angle #alpha#, its enantiomer will rotate light in a direction #-alpha#. Sometimes, the enantiomer is not available for comparison; for instance the sugar that you put on your breakfast cereal is a very common chiral material, and literally mega-tonnes of it are produced, not only chemically pure but OPTICALLY pure. Left-handed sugars, (the same structure but the mirror image) CAN now be produced with effort. It would not be metabolized, because our enzymes, themselves with a specific handedness, are tuned to a specific geometry......

So in closing, rotation of plane-polarized light by chiral molecules is a phenomenon that you just have to accept. If you do experiments in the lab, ask for the insight of your TA; they are usually very experienced, and willing to share their knowledge.