Page 147 - Fundamentals of Light Microscopy and Electronic Imaging

130 PROPERTIES OF POLARIZED LIGHT

optic optic
axis axis

O O
E E

B : n >n o B : n <n o
e
e
+
–
Figure 8-9
The relation of O and E wavefronts in specimens showing positive and negative
birefringence.

biological materials, magnetic permeability is close to 1 and can be discounted; how-
ever, for materials with reduced magnetic permeability (metallic films), this would not
be the case. The dielectric constant is related to the refractive index n by the simple rela-
tionship
2
n .

Therefore, in crystals having a particular lattice structure of atoms and molecules, the
value of n is different in different directions, depending on the orientations of chemical
bonds and electron clouds around atoms and molecules. The short axis (slow axis) of the
wavefront ellipsoid corresponds to the axis defining the highest refractive index value;
the long axis (fast axis) corresponds to the axis having the lowest refractive index value.
The ellipsoid describing the orientation and relative magnitude of the refractive index in
a crystal is called the refractive index ellipsoid or simply index ellipsoid.

BIREFRINGENCE IN BIOLOGICAL SPECIMENS

We have seen that the velocity of light during transit through a specimen is retarded by
perturbations and interactions with electrons in the transmitting medium. The suscepti-
bility of an electronic configuration to distortion is termed its polarizability. The more
polarizable the electronic structure of a molecule, the more extensively light can inter-
act with it, and, thus, the more slowly light is transmitted. In the extreme case, the inter-
action may be so strong that the specimen absorbs quanta of light.
Most chemical groups are asymmetric with respect to polarizability. For example, the
electrons of carbon-carbon bonds in molecules with long carbon chains are most easily
displaced along the direction of the bond (Fig. 8-10). A structure containing parallel chains
of carbon-carbon bonds such as the cellulose polymers in cellophane is most polarizable
2
in a direction parallel to the carbon chains. Since n , the velocity of light is lowest and
the refractive index is highest in this direction. Therefore, when long carbon chain mole-
cules are regularly oriented in biological structures as they are for example in cellulose
fibers comprising a plant cell wall, the polarizability of the structure as a whole varies with
its orientation in the illuminating beam of polarized light. As shown in Chapter 9, the ori-
entation of the wavefront and index ellipsoids and the sign of birefringence of the speci-
men can be determined with the help of a compensator such as a full waveplate.