Light: Polarimetry: Using polarized light to study matter
Polarimetry measures the extent to which a substance interacts with
Plane Polarized Light. Plane Polarized Light is light that consists of
waves that vibrate in one plane, either vertically or horizontally.
Some materials will rotate Plane Polarized Light either to the
Right or to the Left. If a substance rotates Plane Polarized Light
to the Right or Left, it is called optically active.
Light that rotates to the Left, called Left Circularly Polarized Light is illustrated above. |
Light that rotates to the Right, called Right Circularly Polarized Light is illustrated above. |
Below is Java applet that shows how a material (represented by square labeled as "sample") can change the
polarization of transmitted light.
The sample can basically change the polarization through two mechanisms. The first occurs when the sample
absorbs the two polarization of light differently. This is called dichroism, and can apply to difference in
absorption for x and y linearly polarized light (linear dichroism) as well as the difference in absorption for
left and right circularly polarized light (circular dichroism). The second mechanism is due to a difference in
index of refraction for the two polarization components. This causes one component to travel faster through the
sample than the other component, resulting in a phase shift between the two component when they exit the
sample. This is called linear birefringence if the shift occurs between the two linearly polarized components
and circular birefringence if the shift occurs between the two circularly polarized components.
In the simulation below, begin in the linear polarization basis and note what happens to the transmitted
light when the amplitudes and phases of the transmitted linear polarization components change. The
incident light
has equal components along the x and y directions. If for example one applies strain along the x axis, one can
change the optical properties for light polarized along that axis compared with light polarized along the
y-axis.
Now switch to the circular polarization basis and note what happens to the transmitted
light when the amplitudes and phases of the transmitted circular polarizations are change. Applying a
magnetic field along the direction of the light propagation is one way to change the sample's optical
properties for left and right circularly polarized light. What happens if
only the phases are changed? What happens if only the amplitudes of the transmitted circular components are
changed? If the sample completely absorbs left hand circularly polarized light, what type of light polarization
would be transmitted?