PLM Fundamentals:               

The Polarized Light Microscope  

Many different fibers have been used in the manufacture of building materials. Far more common than asbestos are materials such as cellulose, animal hair, nylon and fiber glass. If asbestos is present, it may be present in small amounts, but by law a material is considered potentially dangerous if it contains more than 1% asbestos by weight.

Laws mandate the testing of materials in public buildings and structures that are being bought/sold. Vast sums of money are spent in America each year to analyze building materials for asbestos, but what does it mean to analyze for asbestos? How does a geologist determine if a material is asbestos bearing? If asbestos is present, how much is present?

Essentially the first step in the process of analyzing building materials for asbestos is the use of the POLARIZED LIGHT MICROSCOPE (PLM).

What Is A Polarized Light Microscope?

The PLM is a basic tool of the geologist. It exploits the fact that most minerals are transluscent (i.e., if the mineral grain is thin enough then the light will pass through). When using a polarized light microscope an analyst examines light that passes through a given sample and has interacted with the internal structure of the mineral grains. When the light emerges from the sample it has been altered due to interactions within the mineral grains. Each mineral is unique in its composition and/or structure so each mineral has a unique affect on light when it passes through it. Thus interpreting the emergent light allows a geologist to identify minerals with great accuracy.

Your First Look at the PLM

The Polarized Light Microscope (or petrographic microscope) is a complex piece of machinery.
It allows the operator (petrologist) to manipulate light in several different ways and observe the results after the light has interacted with the mineral sample on the stage.
In the analysis of asbestos we only use a fraction of the abilities and tools of the PLM. 
Below are two pictures of a polarized light microscope, complete with labels on the critical components with respect to this course. It is important that you familiarize yourself with these parts and where they occur with respect to each other.
The parts labeled above will be referred to throughout the discussion of the polarized light microscope.
The relative position of the various parts is important in understanding the behavior of light through the microscope and the mineral samples. A general schematic of the pathway of light in the PLM is provided below: 

DO NOT spend a lot of time right at the beginning memorizing these pictures! Instead, return to this page each time you are introduced to a new piece of the PLM and familiarize yourself in its position and appearance,

© 2001 Wayne G. Powell