The Polarizing Lens Microscope

Reticle Scale

Objective 4X 10X 40X

Scale Bar (mm) 2.5 1.0 0.25

Lg. Divisions (mm) 0.25 0.1 0.025

Sm. Divisions (mm) 0.05 0.02 0.005

1. You will need to use the reticle scale to determine the thickness of the following fibers:

a. Dynel:

b. Dog Hair:

c. Human Hair:

2. You will need to measure the diameter of at least 10 particles of each of the following materials. Include an average and the range for each slide.

a. Potato starch

b. Corn starch

c. Rice starch

Crystal morphology

Acicular crystal habit

a. Calcite

b. Crocidolite

c. Biotite

3. Draw a sketch of the following minerals and describe the crystal habit of each one.

Index of refraction: a measure of how much the speed of light is reduced inside a medium. For example, typical glass has a refractive index of 1.5, which means that light travels at 1 / 1.5 = 0.67 times the speed in air.

Relief: is a qualitative measurement of the degree to which mineral grains stand out from the mounting medium. If the refractive index of the mounting medium and the particle is not the same, the light is refracted on passing from the mounting medium to the grain.

The approximate differences of the index of refraction between the mounting medium and the particle is high relief when > 0.12, moderate relief when between 0.12 and 0.04 and low relief when < 0.04

Relief of anisotropic particles may change as the stage is rotated in plane light.

Plane light

Polarized light

High relief

Low relief

If the index of refraction of the particle is greater than the mounting medium, the particle has a positive relief and if it is lower the particle has a negative relief. In both cases, the particle will appear to stand out.

4. Determine the relative relief (high, moderate, low) of the following particles. Unless otherwise indicated, the particles are mounted in a meltmouth media with a refractive index of 1.662.

a. Radiolaria

b. Quartz

c. Corundum

d. Halite

Analyzer (upper polarizer) -- a polarizing prism located above the microscope stage, between the objective lens and the eyepiece. This restricts the transmission of light vibrating perpendicular to the polarizer. The analyzer can be slipped in or out of the light path or rotated for partially crossed polarized light. Light passing through the polarizer will not pass through the analyzer unless the vibration direction of the light is changed between the two prisms. Anisotropic minerals can perform this deed.

Polarizer (lower polarizer) -- a polarizing prism located beneath the microscope stage (between the light source and the object of study). This restricts transmission of light to that vibrating in only one (N-S) direction. Some microscopes have a different orientation direction. In effect, it plane polarizes the incident light beam.

How is light polarized and how does this help us identify specific minerals?

Becke line: the bright halo near the edge of a transparent particle immersed in a medium. The halo moves with respect to that edge as the focal plane of the microscope is changed. The Becke line moves when the distance between the objective of the microscope and the preparation is changed. *The Becke line will always move toward the higher refractive index medium when the distance is increased and will move toward the lower refractive index medium when the distance is decreased from the point of critical focus.

nglass >nmedium nglass < nmedium

Increase the distance between the objective and the sample…

Anisotropic particle: has more than one index of refraction.

Isotropic particle: has only one index of refraction.

Uniaxial particle: has two indices of refraction.

Biaxial particle: has three indices of refraction.

5. Use the Becke line test to determine if the following particles have and index of refraction greater than, less than or nearly equal to 1.662.

a. Ground glass (isotropic)

b. Quartz (uniaxial)

c. Olivine (biaxial)

d. Human hair (uniaxial)

e. Triacetate (uniaxial)

Pleochroic minerals change color as the stage is rotated when the sample is observed in plane light. The color changes because the slow and fast rays (corresponding to two different indices of refraction) are absorbed differently as they pass through the material and therefore have different colors.

5. Determine if the following particles are pleochroic (yes or no). If they are pleochroic, describe the colors when the stage is rotated.

a. Feldspar b. Crocidolite

c. Cigarette ash d. Amphibole/hornblende