Experiments in Crystal Optics

Hans Dieter Zimmermann
,
Aarhus Universitet
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Summary

The purpose of the experiments below is to impart an intuitive understanding of the interaction between light and crystals and, thus, of optical crystallography. This will help to demystify what is seen in the polarizing microscope, and will better prepare the student for the introduction of optical indicatrices as 3-D models to describe the directional dependence of light velocities, and thus refractive indices in anisotropic crystals.

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Context

Audience

This activity is designed for an undergraduate required course in mineralogy and is generally for sophomore or junior level students.

Skills and concepts that students must have mastered

The following demonstrations and exercises are designed for use in an undergraduate course in mineralogy or optical crystallography. If students are not familiar with elementary optics, I suggest a brief explanation, or recapitulation, of (a) Snell's Law (i.e. the Law of Refraction: rays at boundary surfaces, index of refraction, speed of light in matter) and (b) polarization of light (unpolarized light, polarized light, polarizer, analyzer, plane of polarization). Furthermore, students should have a basic knowledge about crystal systems, crystallographic axes, etc.

How the activity is situated in the course

This activity is a stand-alone exercise, but is part of a larger volume of classroom and laboratory activities from "Teaching Mineralogy," a workbook published by the Mineralogical Society of America, Brady, J., Mogk, D. W., and Perkins, D., (editors), 1997,406 pp.

Goals

Content/concepts goals for this activity

The purpose of the experiments below is to impart an intuitive understanding of the interaction between light and crystals and, thus, of optical crystallography. This will help to demystify what is seen in the polarizing microscope, and will better prepare the student for the introduction of optical indicatrices as 3-D models to describe the directional dependence of light velocities, and thus refractive indices in anisotropic crystals.

Higher order thinking skills goals for this activity

Other skills goals for this activity

This activity involves operating analytical equipment.

Description of the activity/assignment

The classic physical optics textbook approach to double-refraction starts from Huyghens constructions of wave fronts and from the optical indicatrix. Optical indicatrices are useful for a systematic description of optical properties in crystals, but students do not usually consider them an easy subject, and, therefore, shy away from optical crystallography. This is unfortunate since a basic understanding of optical crystallography is prerequisite to a correct interpretation of phenomena observed with the polarizing microscope, the most commonly used tool for the detailed study of rocks.
Generally, students are comfortable with simple optical terms like reflection and refraction, while it is uncommon that they actually have seen double-refraction and noticed that crystals polarize light. Many have an unnecessarily complicated idea about vibration directions, interference colors, and interference figures; they assume such phenomena always require a microscope to observe. This is not so. Students well trained in thin section microscopy are often surprised that interference figures can be made visible macroscopically.
The purpose of the experiments below is to impart an intuitive understanding of the interaction between light and crystals and, thus, of optical crystallography. This will help to demystify what is seen in the polarizing microscope, and will better prepare the student for the introduction of optical indicatrices as 3-D models to describe the directional dependence of light velocities, and thus refractive indices in anisotropic crystals.

Determining whether students have met the goals

Students have successfully met the goals of this activity by completing the experiments involved and demonstrating an increased understanding of crystal optics.

More information about assessment tools and techniques.

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Other Materials

Supporting references/URLs

Brady, J., Mogk, D. W., and Perkins, D., (editors), 1997, Teaching Mineralogy, a workbook published by the Mineralogical Society of America, 406 pp.

Bretschneider, E. & Scholz, O. (1974) Die Physik in Versuchen - Optik. PHYWE Schriftenreihe. 14. Aufl., Goettingen, 1974.


Dowty, E. ATOMS, Computer Program for Displaying Atomic Structures Macintosh Version 1.2, 1992.

Sears F.W., Zemansky M.W. & Young H.D. College Physics, 7th ed. Reading, Mass., 1991, 1060p.

Zimmermann, H.D. Polarisationsmikroskopi. Copenhagen, 1989, 350p.

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