Linear polarization.

For a pure substance in solution, if the color
and path length are fixed and the specific
rotation is known, the observed rotation can be
used to calculate the concentration. This usage
makes a polarimeter a tool of great importance
to those trading in or using sugar syrups in
bulk.
In the presence of an applied magnetic field, it
is possible for samples of all compounds to
exhibit optical activity. A magnetic field aligned
in the direction of light propagating through a
material will cause the rotation of the plane of
linear polarization. This Faraday effect is one of
the first discoveries of the relationship between
light and electromagnetic effects.
Optical activity or rotation should not be
confused with circularly polarized light.
Circularly polarized light is often presented as a
linear polarization rotating as the light
propagates. However, in this picture, the
polarization completely rotates in a length equal
to the wavelength (roughly one micrometer for
visible light), and it can happen in vacuum. In
contrast, optical activity occurs only in a
material, and a complete rotation occurs in a
length of millimeters to meters, depending on
the material.