happens when a beam of ordinary light passes through certain crystals?
Atoms in a crystal are arranged in a large number of parallel slots.
Light will pass through both crystals when their slots are parallel,
but will be completely cut off if the slots are crossed.
A single crystal will then hold back all
the vibrations except the one that is lined up with its own grain.
A beam of light whose vibrations are thus confined to one direction
is said to be plane-polarized. This setup also shows that light
waves are transverse. Longitudinal waves cannot be polarized.
An invention by Nicol can be used to produce
and detect polarized light. This is known as Nicol prism. The prism
is held in front of the beam of light and is rotated. If the beam
is plane polarized the light seen through the Nicol Prism varies
in intensity and none passes through at one position of the prism.
Polaroids are used in many practical applications
of polarized light. For example they are used in sunglasses to reduce
the intensity of incident sunlight and to eliminate reflected light
or glare from the road.
Polarized light can be used to find out
just how the stresses are distributed in machine parts. A model
of the part is made out of plastic and subjected to the kind of
stress the original would get in actual use. When viewed by polarized
light., color bands appear which show the exact stress pattern in
Remember the Nicol Prism? It is used in
a saccharimeter. Saccharimetry is the measurement of concentrations
of sugar. Due to the molecular structure of the sugar, these solutions
rotate the plane of polarization of plane-polarized light as the
light passes through. The rotation of the plane of the polarization
when the incident light is viewed may be right-handed (clockwise)
or left-handed (anti-clockwise).
Polarimetry is the science concerned with
the angle of rotation of plane-polarized light. It is important
in chemistry since many chemical compounds are optically active;
they have the power of rotating the plane of polarization of a beam
of polarized light. The phenomenon occurs when the molecular structure
of the compound lacks symmetry so that the molecule and its mirror
image are not superimposable. Polarimetry has important applications
in the sugar industry, since sucrose is far more optically active
than many of the more common impurities and so polarimetry can be
used to measure the purity of sugar.
Polarimetry is also used to characterize
and distinguish stereoisomers, which are compounds with the same
composition and structure, but different configurations of atoms
within the molecule. Two well known streoisomers are tartaric and
racemic acids.Tartaric acid rotates the plane of polarized light
to the right while racemic acid is optically inactive, suggesting
different symmetries within the two molecules. While these acids
differ in many other observable properties, it is often the case
that stereoisomers can only be distinguished by polarimetry and
by their reactions with other optically active substances.