Question

Why are chiral molecules optically active?

Answer 1

Not only chiral molecules are optically active.

Optically active means they rotate the plane of polarization. Either to the right (`D`) or to the left (`L`).

If we have a molecule with a chiral (asymmetrical) `C`-atom in it, it is possible to have two versions of the molecule, one being the mirror image of the other.(try this with the balls-and-sticks). Just like a pair of gloves.

Whereas one of the stereo-isomers (as they are called) rotates the plane of polarization one way, the other one will mirror this, and rotate it the other way.