Page 138 - Fundamentals of Light Microscopy and Electronic Imaging

VECTORIAL ANALYSIS OF POLARIZED LIGHT USING A DICHROIC FILTER 121

• Using two crossed polars positioned on a light box, examine a piece of glass
such as a microscope slide and a sheet of cellophane, while rotating them
between the polars. The glass does not interact with polarized light and
remains essentially invisible. Such materials are said to be optically isotropic.
However, rotation of the cellophane sheet through 360° reveals four azimuths
separated by 90° at which the entire sheet looks very bright. The unique abil-
ity of cellophane to interact with polarized light is due to the presence of
aligned parallel arrays of cellulose molecules and birefringence, which is
described later in the chapter. Materials of this type are said to be optically
anisotropic. All objects suitable for polarization microscopy exhibit some
degree of molecular orientation and optical anisotropy.

POLARIZATION BY REFLECTION AND SCATTERING

Polarized light is also produced by a variety of physical processes that deflect light,
including refraction, reflection, and scattering. Light reflected from the surfaces of
dielectric materials is partially linearly polarized, with the E vectors of the reflected
waves vibrating parallel to the reflecting surface and the extent of polarization increas-
ing with decreasing angles of incidence. For light incident on a transparent material
such as water or glass, there is a unique angle known as Brewster’s angle, at which the
reflected waves are completely plane polarized (Fig. 8-4). For the simple case of a beam
of incident light travelling through air (n 1), the critical angle is given as

tan n.

For water (n 1.33) and glass (n 1.515) the critical angles are 53° and 57°, respec-
tively. As an interesting note on reflection polarization, manufacturers of Polaroid sun-
glasses mount sheets of polarizing material in the frames with the transmission axis of
the Polaroids oriented perpendicularly in the field of view. Bright reflections off hori-
zontal surfaces, such as the roofs of cars or water on a lake, are very efficiently blocked,
while the random light is partially blocked, since only vertically polarized rays can
reach the eye.
Linearly polarized light is also produced by light scattering. A common example is
the polarization of light in the northern sky caused by the scattering of sunlight by air
molecules. The extent of polarization ( 50%) is readily appreciated on a clear day by
rotating a polarizer held close to the eye. In accordance with principles of scattering,
polarization is maximal at an angle 90° from the sun. For additional information on
polarization by refraction, reflection, and scattering, see the interesting discussions by
Minnaert (1954) and Hecht (1998).

VECTORIAL ANALYSIS OF POLARIZED LIGHT
USING A DICHROIC FILTER

The Polaroid sheet just described commonly consists of a film of parallel arrays of lin-
ear polyvinyl alcohol molecules with embedded polyiodide microcrystals (H-ink)