Illustrated Glossary of Organic Chemistry

Optically active: A substance which has optical activity, i.e., a substance which rotates the plane of plane polarized light.

(R)-2-Hydroxybutanoic acid
One stereocenter
Optically active
   
Tartronic acid
No stereocenter
Optically inactive




(+)-(2R,3R)-Tartaric acid
Two stereocenters
Optically active
   
(2R,3S)-Tartaric acid
A meso compound
Has stereocenters, but is optically inactive