Exploring the Uniaxial Indicatrix

Sarah Penniston-Dorland
,
University of Maryland
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Summary

This activity gives the students a 3-dimensional perspective on the uniaxial indicatrix, its various parts, and the relationship between different orientations of the indicatrix and the corresponding optical properties of the mineral.

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Context

Audience

Undergraduate required course in Optical Mineralogy

Skills and concepts that students must have mastered

The student must have been introduced to the concepts of vibration directions of light and the wave normal of light. The student must also understand the concept of index of refraction. The student should have been introduced to the concept of uniaxial minerals and the geometrical concept of an indicatrix which is used to describe the variation in the indices of refraction depending on the orientation of the mineral relative to incoming light. The student should also have been introduced to the concepts of crystallographic axes and optic axes and to birefringence.

How the activity is situated in the course

This is a single exercise designed for one class period in which students can reinforce the concepts of the indicatrix for uniaxial minerals. I have a similar exercise later in the semester where the biaxial indicatrix is explored.

Goals

Content/concepts goals for this activity

The student should be able to understand the geometrical concept of an indicatrix which is used to describe the variation in the indices of refraction depending on the orientation of the mineral relative to incoming light.

The student should be able to relate the orientation of the indicatrix to the observed birefringence for a uniaxial mineral.

Higher order thinking skills goals for this activity

relating geometric aspects of minerals to observations made using a petrographic microscope

Description of the activity/assignment

This exercise is designed to give the students a hands-on experience in which they can relate different orientations of an indicatrix to the different birefringence observed for each orientation. Each student is given a piece of fruit that has the shape of a uniaxial indicatrix - either a kiwi (uniaxial positive) or a tangerine (uniaxial negative). The student follows a set of instructions to insert toothpicks to represent the different indicatrix axes and to draw on the fruit using a permanent marker the circular and principal sections. Then the student is asked a series of questions asking them to determine the birefringence of the mineral for different orientations of the fruit.

Determining whether students have met the goals

There is a place on the students handout after each step for the instructor to check that they have completed each step correctly.