Page 136 - Fundamentals of Light Microscopy and Electronic Imaging

THE GENERATION OF POLARIZED LIGHT 119

360 0

270
90

180
(a) (b)

Figure 8-2
Random vs. linearly polarized light. The drawings show two beams of light, each containing
several photons, as they would appear in cross section, looking down the axis of the beam.
Refer to Figure 2-2 for orientation. (a) Random light: The E vectors of the waves are
randomly oriented and vibrate in all possible planes. (b) Linearly polarized light: The E
vectors of all waves comprising the beam vibrate in a single plane. The angle of the plane of
vibration of the E vector relative to the vertical reference line is designated . The angle of tilt
is called the azimuthal angle.

ing the orientation of polarized light whose plane of vibration is not known. Two linear
polarizers—a polarizer and an analyzer—are incorporated in the optics of a polarizing
microscope.

Demonstration: Producing Polarized Light
with a Polaroid Filter

A Polaroid sheet, or polar, is a polarizing device that can be used to demonstrate
linearly polarized light. The sheet has a transmission axis, such that incident
waves whose E vectors vibrate in a plane parallel to the axis pass through the fil-
ter, while other rays are absorbed and blocked (Fig. 8-3). Because of its unique
action, the Polaroid sheet can be used to produce linearly polarized light or to
determine the plane of vibration of a polarized beam whose orientation is not
known. Used in these ways, the sheet is then called, respectively, a polarizer or an
analyzer. To become familiar with polarized light, perform the following opera-
tions using a pair of Polaroid sheets:

• Place two polars on top of each other on a light box and rotate one of the
polars through 360°. At two azimuths separated by 180°, light transmission
reaches a maximum, while at two azimuths separated from the first two by
90°, transmission is substantially blocked, and the field looks black. In the
first case, light is transmitted because the transmission axes of the two polars
are parallel, and all of the light transmitted by the first filter passes through