Enantiomers are optical isomers that are non-superimposable mirror images about a chiral centre.

For example, the enantiomers of lactic acid are shown below.

A chiral centre is a carbon atom in a molecule bonded to four different atoms or groups.

Chiral centres are often indicated on formulae by an asterisk (*).

Enantiomers rotate plane-polarised light in opposite directions; one rotates it clockwise, and the other rotates it anti-clockwise.

A molecule is considered optically active if it has the ability to rotate the plane of polarised light due to its chiral structure.

A racemic mixture is an equal mixture of two enantiomers of a chiral compound.

A racemic mixture is also called a racemate.

Racemic mixtures are optically inactive because the rotations caused by the two enantiomers cancel each other out.

Since there are equal amounts rotating clockwise and anti-clockwise, the net rotation is zero and no optical activity is observed.

Optical isomerism is a type of stereoisomerism where isomers have the same structural formula but different arrangements of atoms in space.

Optical isomers can be distinguished by their ability to rotate plane-polarised light in opposite directions. One optical isomer rotates it clockwise, while the other rotates it anti-clockwise.