A linear polarizer is a device which transforms any type of incident light into light which is linearly polarized. The polarizer accomplishes this by absorbing the component of the light which is polarized perpendicular to its transmission axis, leaving only light polarized parallel to its axis to pass through. Ideally, if unpolarized or natural light was passed through a linear polarizer, one half of the initial intensity would pass through. A quarter-wave plate creates elliptically or circularly polarized light by allowing both x and y components of the electric field to pass, but at different speeds. Using a birefringent material with different indices of refraction for different states of polarization, one component of the light will slow down and take longer to transverse the material than the other component. The two parts recombine at the other side of the material, but out of phase, causing a constantly changing direction of polarization.
When light is scattered, polarization is affected. This can be easily seen in the sky during the daytime when natural light from the sun scatters off molecules in the air near the horizon to reach your eye. If a polarizer were rotated between your eye and the distant sky, the intensity would change dramatically. When light is directed onto small particles like the molecules in the air, it can be re-radiated in any direction except along the plane of the initial electric field. If the natural sunlight is always represented by vectors in the $x$ and $y$ direction, with the $x$ direction being in the same plane as your line of sight, the light which reaches your eye will be plane-polarized in the $y$ direction \cite{hecht}.
In the present experiments, individual photons scatter off small
particles repeatedly until they finally leave the scattering substance
and are focused towards the CCD camera which analyzes them. Because
the incident light in polarized, the intensity of the outgoing light
will not be uniformly distributed in all areas of the sample. A
dipole pattern is expected to be captured by the camera when linearly
polarized light is directed onto a uniform substance. The presence of
an axis perpendicular to two symmetrical areas of high intensity near
the center of the image is caused by the inability of particles in the
scattering medium to re-radiate light along that one plane. However,
this occurs near the center of the image, as areas further away
represent photons that have been scattered so many times that their
polarization is lost, or decided by chance.
[Title Page]
[Introduction]
[The Mueller Matrix]