optical isomeria

The word 'optical' indicates that molecules of this type of isomeria have special properties connected to optics.
Every organic substance having one or more C atoms connected to four different atoms or atom groups, had 'optical activity': the can turn a vibration plane of monochromatic light, to the left or to the right.
This phenomena can be measures with a polarimeter. The atom connectedto four different groups acts like that and is called an "optic active centre".

      D-alanine          L-alanine
they are each others mirror image
D-alanine          D-alanine
they are equal,
but not each others mirror image
More information about stereochemistry can be found in websites like: shapes in organic chemistry