Geology Reference
In-Depth Information
Snow flake
c - axis
Unpolarized
light
No light
(black)
Plane-polarized
D light
Analyzer
(cross position)
Polarizer
Figure 6.3 Optical components of a standard large‐field polariscope with its polarizer and the analyzer in cross
position; a snowflake with its c axis parallel to the direction of light is inserted here to show that it will look black,
irrespective of its rotation (N. K. Sinha, unpublished).
The double‐headed arrows on the two polars (or polarizers)
in Figure 6.3 indicate their orientations or pass directions.
If a snowflake or a plate of single crystals of ice, with c
axis parallel to the direction of propagation of light, is
placed between the polarizer and the analyzer in cross
position, as shown in Figure 6.3, it will appear as black.
This is a position of “extinction” for the snowflake or the
crystal and is used extensively in determining orientation
fabric diagrams for ice using polariscopes equipped with
Rigsby universal stages as presented in Bader , [1954] and
Langway [1958]. It is appropriate to mention here that
similar arrangement of a thin section of quartz posi-
tioned between and parallel to crossed polarization was
defined as the “standard photometric configuration” by
Martinez [1958]. The illustration also explains why beau-
tiful stellar flakes of snow placed flat on polariscopes
exhibit no colors and may disappoint the enthusiastic
explorer.
Descriptions of a Rigsby‐type universal stage and a
large‐field polariscope are given later in this chapter
in  section  6.3.1. Polariscopes are made by making use
of  commercially available large sheets of polarizing or
Polaroid filters. It is preferred to cut the sheets as rectan-
gular pieces with their lengths parallel to the pass direc-
tion or the direction of polarization (as illustrated in
Figure  6.3). Most camera lenses can also be fitted with
the commercially available polarizing filters with rotating
mounts. In case of a polariscope equipped with linearly
polarizing sheets, the outer rings holding the two polars
should be marked with lines indicating their pass direc-
tion. These markings allow a very quick method of set-
ting the two polarizers either in cross or parallel position.
In the field of glaciology the use of cross polarizers is
very popular and used almost universally for photo-
graphic illustrations of ice structure.
6.1.2. Birefringence or Double Refraction of Ice (Ih)
There are essentially three divisions of crystals from an
optical standpoint. They are: isotropic (isometric), uni-
axial (tetragonal and hexagonal), and biaxial (orthorhom-
bic, monoclinic, and triclinic). Ordinary ice (called Ih)
belongs to the hexagonal system and consists of oxygen
(O) and hydrogen (H) atoms. The stoichiometric compo-
sition of water has been investigated by numerous inves-
tigators over a very long period time and been documented
thoroughly in many reports and topics. The ratio of the
combining volumes of oxygen (O 2 ) and hydrogen (H 2 ) at
0 °C and a pressure of 0.76 mmHg is O 2 /H 2 = 1/2.00288.
Essentially 2 atoms of H are attached to 1 atom of O in a
molecule of water. In natural water, 1 atom of D 2 (deute-
rium, denoted by D is a stable isotope of hydrogen) is
also present for about 6500 atoms of H 2 .
The atomic and the molecular structure of water in all
its phases (liquid and solid) have intrigued scientists for
many centuries. Although the chemical formula of water
is very simple, its atomic structure, depending on tempera-
ture and mechanical or electrical forces, is extremely com-
plicated and difficult to study. It was known for a long
time that a molecule of liquid water consists of one oxy-
gen (O) atom and two hydrogen (H) atoms. The H atoms
are not symmetrically placed around the O atom in a mol-
ecule of water. The angle between the two O‐H bonds
is slightly greater than 90°, close to 104.5° [ Pounder , 1965,
Chapter  5, p. 62]. This makes it a polar molecule with a
permanent electric dipole moment. When a uniform elec-
tric field is applied to water, it becomes slightly “doubly
refracting” or “bi‐refringent” like an optically positive uni-
axial crystal with its optic axis in the direction of the field.
This is known as the Kerr electrooptic effect for water
[see Dorsey , 1968, p. 381]. In case of a uniaxial crystal, it's
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