TEACHING CONCEPTS OF POLARIZED LIGHT MICROSCOPY WITHOUT USING A POLARIZING LIGHT MICROSCOPE

We depart slightly from the theme to discuss two things: the polarizing light microscope (PLM), an instrument that is being taught less frequently, but still perhaps has given us more information about rocks and minerals than all instruments combined; and how to teach the use of this instrument without actually using it. While advances in instrumentation have been a boon to the earth sciences, none of these alone can replace the information and the critical thinking skills gained from use of the PLM. With this inexpensive instrument a student, correctly taught, can gain information about the chemistry, structure, and identification of minerals; also, they can gain employment! One challenge of the PLM is that it is not computerized. The user must correctly interpret what they see based on a properly taught set of observation skills. However one major issue in learning to use the PLM arises because what we see comes through the eyepiece, and this microscope view may be unfamiliar to most students. One solution is to find examples in the macro world students can use to understand the micro world. These examples also emphasize the principles of light interaction with minerals without the students worrying about how to use the PLM first.

Calcite rhombs have been used to aid in the understanding of how minerals polarize light at the macro scale, and should continue to be used. Large flakes of muscovite can be placed between two polarizers on a light table or overhead to show extinction and retardation. With the muscovite sandwiched between the polars and 45 degrees off extinction, bring the assemblage as close to your eye as possible, and you can see a perfect acute bisectrix interference figure. Tilting this, you can image the figure in different directions. Another way to see interference figures is to use half a ping pong ball. This is placed on a polarizer which is then placed on the muscovite. The entire assemblage is placed on the stage of a PLM with conoscopic light. This illustrates that interference figures are really 3D constructs, not just 2D as appears in the microscope. Interference plates can also be made of tape or plastic and used to show subtraction and addition. Becke lines also are much easier to understand in the PLM once they are seen on a larger scale. We will explain in detail how to make these demos on our poster and with computer animations.