Polarized Crystals
Richard Haynes
Beautiful iridescent crystals of many materials
can be seen through a microscope equipped with polarizing
film filters. Seeing such crystal colors depends upon two
properties of light: polarization and birefringence.
A brief explanation for each follows. No great detail regarding
either property will be covered here as this information is
easily obtained from many sources. (The reference section
offers a few suggestions.)
.
Light exists in two forms: discrete packets or photons and
waves. Here we are concerned with waves. Light, as a wave,
vibrates in all directions, or planes, that are at right angles
to the path of the light. If the path passes through a film
capable of allowing only vibrations in one particular plane,
the light that emerges is called plane polarized,
or just polarized. If light passes from air through
certain other transparent materials, such as quartz, the light
bends, or is refracted, into two paths (rays), each polarized
with its vibrations at right angles to the other and each
moving at a different velocity. This is double refraction,
or birefringence.
Figure 1. Birefringence.
The path of light in a polarizing microscope
passes through the first polarizing filter, then the specimen
crystal, then the second polarizing filter, which is called
the analyzer filter, and, finally, the eyepiece.
After passing through the polarizing filter, the light is
extinguished (absorbed) to some degree by the analyzer filter.
The angle of the polarizer to the analyzer where total light
extinction is reached is 90 degrees. Usually, total blackness,
or absence of light, is not reached in a typical microscope.
Because the specimen crystal has also bent the light, rays
passing through the analyzer arrive at the eye at differing
velocities…or patterns and colors. Such crystals are
anisotropic and birefringent.
The microscope must be capable of accepting
a polarizing film in its light beam (after the objective)
and a second polarizer film (the analyzer) before the eyepiece.
Most student and better microscopes are capable of handling
such films. One of the two polarizing films pieces must be
rotatable to achieve the 90-degree extinction angle necessary
to see the full crystal colors. Suitable polarizing film can
be purchased from a number of sources. Even old polarizing
sunglasses with plastic lenses might serve if they can be
cannibalized. See references.
Many household products, including certain
foods, pain relievers, pharmaceuticals, vitamins, moth crystals,
darkroom chemicals, some dairy products, etc., appear as beautiful
crystals under polarized light. Obviously, some of these materials
must be handled with care. Sugar is a crystalline substance
that is colorful, safe, and easy to make as slides and view.
Recently I created and photographed preparations of laboratory
grade sugar (dextrose) and five different super market sugars:
granulated, raw washed, “glittering,” confectioners
(powdered) and brown sugar.
Several techniques for producing crystals
on slides to view in polarized light are available. Among
these is the melt method, where the material is placed on
a glass slide, held over a low flame, gently melted and then
allowed to cool with a thin cover glass over the melt. Only
materials that will not decompose or otherwise change forms
are suitable for melting. Sulfur makes beautiful crystals.
Another method is to make a solution of the
specimen, add a drop or two to a slide, and then place a cover
glass over it. Allow the soluble material to slowly form crystals
under the cover glass.
For the sugar crystals, I used a different approach that is
simple, non-quantitative and especially good for quick observations.
A couple of drops (~ 0.1 ml) of distilled water are placed
in the center of the slide, and a very small amount of sugar
(a few crystals) is dropped into the water. This material
is crushed and dissolved with the aid of a glass stirring
rod.
The water must be removed to form the thin
crystals required. I do this by laying the slide without a
cover glass about 20 cm (8 inches) from the face of a 150
watt infrared heat lamp. The water evaporates quickly. I use
no cover glass.
CAUTION: If any solvent other than water
is used, DO NOT EMPLOY THE HEAT LAMP APPROACH! Allow the solvent
to evaporate in the open air.
The slide is put on the stage of the polarizer-equipped
microscope and the light turned on. Using either the 4X or
the 10X objective, the crystal is brought into view and critically
focused. The eyepiece (containing the analyzer) is rotated
until the most brilliant patterns and colors are seen.
Sucrose
First we will examine sucrose.
Figure 2. Sucrose (C = green, O = red, H
= white) C12H22O11.
Figure 3. Representation of monoclinic crystal form.
Figure 4. Granulated sucrose (10X). Figure
5. Raw washed sucrose (10X).
Figure 6. “Glitter” (10X). Figure
7. Confectioners sugar. (10X).
Figure 8. Brown sugar (10X).
The sugars in Figs. 4-8, all sucrose, are
manufactured using slightly different methods. Raw washed,
sometimes called turbinado sugar, is made from cane sugar
that has been partially washed to remove some of the surface
molasses and leaving behind large, blond colored granules.
“Glitter” is a specialty large granulated sugar
that is dyed various colors and used for baking decorations.
(Notice the red specks of dye in Fig. 6.) Confectioners sugar
is very finely ground to a white powder. Brown sugar (from
cane) is sugar crystals coated with molasses and can be made
by either centrifugation of brown sugar crystals or blending
molasses syrup with white sugar.
The varieties in form shown in Figs. 4-8
are due as much to the rapid slide crystallization procedure
as to the slight differences in their manufacture. However,
bubbles of trapped water cover the surface of the brown sugar
in Fig. 8.
Dextrose
We will next look at dextrose.
Figure 9. Dextrose (C = green, O = red, H
= white) C5H11O5;CHO.
Figure 10. Representation of rhombohedral
crystal form
Figures 11 and 12 show dextrose in flat plates.
Figure 11 is similar to the sucrose images. Figure 12 exhibits
entirely different structures. At the left and bottom edges
of the image we see small groups of rhombic crystals, while
the “mountain” ring seems composed of multi-rhombic
crystals. At least four color spectrums are visible, depending
upon the height of the ring. (The out of focus center is coming
toward the viewer into a point.) This is the first such sugar
ring I have seen, and I suspect that during the very rapid
crystallization clumps of rhombic crystals clustered around
a tiny bit of undissolved dextrose, forming the circular structure
(perhaps to save energy?).
Figure 11. Dextrose crystals (10X). Figure
12. More dextrose crystals (10X).
Not only is polarization microscopy interesting,
beautiful and fun but also it can be a very important tool
for studying various inanimate and animate materials.
A final note: The latest U.S. sugar consumption
data [2004] show that we ingest about 100 pounds per person
per year! Something to think about, isn’t it?
References
Book:
Nachtigall, Werner, "Exploring with the Microscope,"
New York, Sterling Publishing, 1996, paper, $14.95 (A very
good general introduction to serious amateur microscopy; covers
most topics.)
Suppliers of
Polarizer Film: (All are reputable but the
author does not endorse any one over the other.)
Carolina Biological Supply
Edmund Scientific Company
Ward’s Natural Science
Microscopy
Information:
Amateur Microscopy
Contains a good four-part series devoted to beginner’s
polarized crystal microscopy
Crystal Gallery
Beautiful pictures of polarized chemicals.
Introductory
Microscopy This Florida State U. site presents possibly
the best overall information on general and specialized microscopy
currently available on the web.
Micscape Magazine
Arguably the finest e-zine on amateur microscopy on the web.
Contains many articles on polarization and other topics.
Nikon
An excellent primer, particularly for identification uses
of polarized light, e.g., natural vs. synthetic fibers, rocks
and minerals, etc. Lots of polarized light technology. 
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