The short answer: any crystal or molecule that can't be superimposed on its mirror image will be optically active.
The long answer:
Circularly polarized light waves twist like a corkscrew in the direction of propagation. The corkscrew
can twist clockwise (as viewed from the light source) or counterclockwise.
Sending equal and opposite clockwise and counterclockwise waves through the same region of space produces plane polarized light, which oscillates in a flat plane.
Consider passing a plane polarized beam of light through a
sample. For many substances, the clockwise and counterclockwise components of the beam are indistinguishable.
Molecules or crystals that can't be superimposed on their own mirror images will interact
with the two components differently, just as a hand fits differently into a left glove or a right glove.
Some asymmetric substances will slow down the clockwise component. Others will slow down the counterclockwise component. Either way, one of the components pulls out in front of the other, and they interfere to produce a plane-polarized beam that has been rotated when compared to the original beam.
The ability of a substance to rotate plane-polarized light is called optical activity.
You can find examples of optically active molecules by looking for certain features that make it impossible for the molecule to be superimposed on its mirror image. For example, if the molecule contains a tetrahedral carbon attached to four different groups, it will be optically active. Most amino acids are optically active for this reason.
Author: Fred Senese firstname.lastname@example.org