Stereoisomerism

Stereoisomerism is a form of isomerism where molecules have the same molecular formula and sequence of bonded atoms (constitution), but differ in the three-dimensional orientations of their atoms in space.
It is split into two major kinds: geometric isomerism and optical isomerism.

Geometric isomerism, also known as cis-trans isomerism, occurs due to restricted rotation of molecules, often as a result of the presence of a carbon-carbon double bond or a ring structure.
Cis isomers are those where the two largest groups are on the same side of the double bond.
Trans isomers, on the other hand, have these groups on opposite sides of the double bond.
Geometric isomers often have different properties and reactivities.
An example of geometric isomerism can be seen in the molecules but-2-ene (cis and trans).

Optical isomerism is a form of stereoisomerism that occurs when a molecule can form two different configurations that are non-superimposable mirror images of each other.
Optical isomers are known as enantiomers.
A defining feature of optical isomerism is the presence of a chiral centre - a carbon atom with four different groups attached.
An example of optical isomerism can be seen in amino acids.
Enantiomers often have near identical physical and chemical properties except for the way they interact with plane-polarised light and react with other chiral molecules.
A solution of one enantiomer will rotate the plane of plane-polarised light to the left (levo or l- isomer) and the other to the right (dextro or d- isomer).
A 50:50 mixture of two enantiomers, known as a racemic mixture, does not rotate plane-polarised light as the two opposite rotations cancel each other out.