Page 134 - Fundamentals of Light Microscopy and Electronic Imaging

CHAPTER
8

PROPERTIES OF POLARIZED LIGHT

OVERVIEW

In this chapter we turn our attention to polarization microscopy and a unique class of
molecularly ordered objects that become visible upon illumination with polarized light.
Figure 8-1 demonstrates the unique ability of a polarizing microscope to reveal molec-
ular order in crystals and starch grains found in plant cell cytoplasm. Polarized light is
also used in interference microscopy, including differential interference contrast (DIC)
microscopy. Although we can observe high-contrast images of ordered objects using a
polarizing microscope, it is remarkable that the eye has no ability in the usual sense to
distinguish polarized light from random light. For this we require special filters called
polarizers, retarders, and compensators. The relationships between the physics of polar-
ized light and images of molecularly ordered specimens are remarkable in their econ-
omy and precision and are well worth mastering. Since the topic of polarized light is
technically demanding, we use this chapter to describe its generation, properties, and
interaction with different objects and optical devices. Our goal is to understand the
working principles of the polarizing microscope, which is described in Chapter 9. Our
reward will be in appreciating how the polarizing microscope reveals patterns of molec-
ular order that otherwise can only be studied using more expensive, technically difficult
methods such as electron microscopy or X-ray diffraction that operate at the resolution
limit of molecules and atoms.

THE GENERATION OF POLARIZED LIGHT

The bulk light from most illuminators used in light microscopy is nonpolarized, the E
vectors of different rays vibrating at all possible angles with respect to the axis of prop-
agation (Fig. 8-2a). In a ray or beam of linearly polarized light, the E vectors of all
waves vibrate in the same plane; the E vectors of beams of polarized light covering an
extended area are plane parallel. Since the plane of vibration of the E vector can occur
at any angle, to describe the orientation of the plane in a beam cross section we describe
the angle of tilt relative to a fixed reference plane designated 0° (Fig. 8-2b). A device
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