Chapter 28: Physical Optics: Interference and Diffraction
Applications

ROCKS & MICROSCOPES

Polarized Light and Optical Activity

Light energy propagates through space as an oscillating pair of electric and magnetic fields. Electric and magnetic fields are vectors and as such have directions as well as magnitudes. The magnitudes of the fields determine the amplitude of the wave and as such they influence the intensity of the light.
The directions of electric and magnetic field vectors determines the polarization of light. Incandescent Light sources such as flames emit unpolarized light. The directions of the electromagnetic fields in unpolarized a light are oriented randomly. (Of course,the electric and magnetic field vectors are always perpendicular to each other.) There are other sources of light, such as lasers, which produce polarized light.
Light can also become polarized on reflection or on passing through a material medium. As light passes through a material medium the energy gets absorbed by the molecules of the media and re-emitted. Sometimes the molecules of the media have a structure that is sensitive to the polarization of light. The material may selectively pass or absorb light with specific directions of polarization. It may also alter the direction of polarization. Such material are called optically active.
Commercially available polarizers are made of optically active materials. They suppress the electromagnetic vibrations except for one preferred direction. If light passes through two such optical filters in succession it gets further attenuated (unless the filters' preferred directions are aligned) or cut out altogether if the filters are oriented at 90 degrees to each other (see sketch on the left).

Birefringence (Double Refraction)

The image on the left shows the view of a page through a crystal of clear calcite (CaCO₃. The crystal was placed on the page as shown in the image on the right. Calcite is birefringent. A light ray passing through a birefringent medium splits into two rays which travel through the crystal at different speeds. Since they have different speeds they have different indices of refraction. Materials with this property are called optically anisotropic. They refract the two rays at different angles (except for normal incidence) and we see a double image.
Not only do the two rays travel at different speeds, they are polarized in different directions. The direction of polarization is related to the orientation of the molecules in the crystal and thus intimately toed to the geometry of the crystal. This fact is of great importance to geologists who use this phenomenon to identify the minerals present in various rock samples.
White light passing through an anisotropic crystal placed between two polarizing filters (called a polarizer and an analyzer) will produce a color patterns as various frequencies get suppressed as the two rays get out of phase.
There are several varieties of anisotropy.
1. The spacing of unit molecules in a crystal is different on at least two directions. Calcite is in this category.
2. Multiples of such crystals in a larger crystal will show additional anisotropy.
3. Long, flat molecules (e.g. organic polymers) will have atomic dipoles arranged in long chains. Electric fields vibrating along the chains get reinforced, electric fields vibrating across the chain get suppressed.
4. A two-phases system, consisting of long or flat particles embedded in a medium of a different index of refraction will show birefringence.
5. Material subjected to stress will show birefringence. An example of this is the glass in the window of your car. The glass is stressed so that it shatters into small pieces rather that long sharp shards.

A Microscope for Geology

A geological microscope has all the parts of an ordinary microscope. In addition it is equipped polarizing filters. A thin slice of a rock sample is placed on the microscope stage. Comparing the colors to a colors on a calibration chart, called the Michel-Levy birefringence scale , the identity of the mineral components of the specimen can be determined. The chart also works the other way; the thickness of a slice of a known compound can be determined form the color. The chart was originally designed to work with standard 30 micrometer sections. It has since found other application, such as determining thickness and composition of carpet fibers.

Some Further Study Links:

Further Study Questions:

Thin layers of transparent materials, such an oil slick on water, often exhibit colorful patterns. Is this phenomenon similar to what was discussed above or is it quite different?

Is it possible to orient a birefringent crystal, such as calcite in the picture on the right, so that the two images coincide? Will the same orientation work for all wavelengths?

People often wear polarizing sunglassed to suppress the glare as sunlight is refelected of shiny surfaces. How does the polarizer in the sunglassed suppress the glare?