Geology Reference
In-Depth Information
the nearest and farthest objects in a scene that appear
acceptably sharp in an image). Polariscopes are com-
monly used to view and photograph ice thin sections
because they utilize the birefrengence property of ice that
reveals its polycrystalline structure. In fact, the optical
characterizations of minerals are made by means of a
special form of microscope known as the “polarizing
microscope” in addition to standard laboratory equip-
ment such as a refractometer, a goniometer, and an axial
angle apparatus [
Rogers
, 1937]. However, polarizing
microscopes can handle only small specimens (<10 mm
width). Therefore, they are not suitable, as discussed ear-
lier in detail, for viewing ice thin sections that are typi-
cally made from ice cores of 10 cm in diameter or ice
blocks of even larger size. Replicas of the top surfaces of
thin sections can also be made and viewed and micropho-
tographed using a scanning electron microscope (SEM).
It produces images by scanning the sample with a focused
beam of electrons that interact with the electrons in the
sample. SEM can achieve magnification between 10× and
500,000× with an ultimate resolution better than 1 nm.
The sample can be a few centimeters wide.
structures are to be examined. They are extremely useful
for examinations of intergranular and intragranular
defects in ice, as illustrated throughout this topic, and the
structure of snow presented earlier in this chapter in sec-
tion 6.2.3. This led to the development of large‐field opti-
cal devices or polariscopes and large‐field universal
stages, such as Rigsby stage, capable of handling sections
of ice cores with diameters of 100 mm or specimens with
dimensions up to 300 mm in diameter as in the case of the
NRC of Canada system to be described later in this sec-
tion. Such devices also allow photographing thin sections
larger than that allowed for recording with usual optical
microscopes.
“Large‐field” polariscopes can be made relatively easily
and cheaply by using commercially available plastic
sheets of polarizers. “Large field of view” is defined here
as large viewing area so that thin sections of ice contain-
ing a large number of grains (minimum of 100) can be
seen and/or photographed at the same time. Another
loose definition could be a polariscope that can handle
thin sections of ice cores with diameters up to, say,
300 mm. Such large thin sections are also compatible if
microstructure properties relevant to wavelength of
microwave signal (10-100 mm) are to be examined.
The transmitted light beam in traditional polarizing
microscopes used to be polarized by making use of Nicol
prisms. Since the Nicol prisms are made from quartz
crystals, the available size of such crystals was the limit-
ing factor for the viewing area of microscopes. However,
the situation changed drastically during the 1930s. Edwin
H. Land championed the properties of dichroic crystals
and patented a rather revolutionary technique in 1929 for
making plastic sheets of polarizers, known as Polaroid
filters. The H‐sheet Polaroid, invented by Land in 1938
[see
Land
, 1951] is made from polyvinyl alcohol (PVA)
polymer impregnated with iodine. The long winding
chains of PVA polymer are stretched, during manufac-
turing of sheets, such that they form an array of aligned,
linear molecules in the material. The aligned PVA mole-
cules become conducting along the length of the chains
when the iodine dopant attaches to them. Consequently,
light polarized perpendicular to the chains is transmitted,
whereas light polarized parallel to the conducting chains
is absorbed.
Plastic polarizing sheets are now used extensively for a
number of wide‐ranging applications, such as sunglasses,
optical microscopes, and liquid crystal displays like the
screens of laptops and TVs. Commercially available PVA‐
iodine filter has very good polarization efficiency (99.9%)
and is almost neutral in color and does not distort the
color significantly. Availability of large PVA panes of lin-
early polarizing filters made it possible to increase the
field of view of polarizing microscopes with diameters of
a few millimeters to large diameters.
6.3.1. Laboratory and Handheld Polariscope
There seems to be a general confusion as to the use of
the term “polarizing microscope” and “polariscope”.
Polarizing optical microscopes have complex optical sys-
tems of condensers and objective lenses and have a very
limited field of view. For this reason, the standard polar-
izing microscopes require small specimens (thin sections).
These microscopes are used for performing petrographic
analysis of minerals, among many other applications. For
such applications, the specimens or slides are small, usu-
ally not more than a few millimeters in diameters. Most
microscopes have specimen holding carriages with
x‐y
movements for handling larger specimens, but only a
fraction of such specimens can be examined at one time.
Polariscopes, on the other hand, are simple optical devices
with large viewing areas that can handle significantly
large specimens compared to those used in optical micro-
scopes. The name polariscope is also referred to the abil-
ity of the instrument to see the “poles” or the optical axis
of individual crystals in a specimen.
Viewing areas (i.e., field of view) of optical micro-
scopes, equipped with polarizers, are usually in the range
of a few millimeters. The size of grains (or individual
crystals) in natural ice is significantly larger (often orders
of magnitude) than those of most other natural crystal-
line materials. The field of view necessary for petro-
graphic studies of natural ice, therefore, has to be large
and capable of handling as many grains as possible. For
this reason, the polarizing microscopes are not very use-
ful for ice unless the subgrain or the intragranular
