In its simplest form an optical microscope takes light reflected from the surface of an object and bends this light by means of lenses so that it forms an enlarged image. If an object allows light to pass through it this transmitted light can be similarly manipulated. Geologists found that by grinding a slice of rock to form a thin section about 1/200" (0.01cm) thick, light could pass through many of the minerals forming it and microscopic examination in this manner yielded much additional information on the rock's composition and structure.
Normal light has waves that vibrate in all directions. Polarised light has waves that vibrate in one direction only. When polarised light encounters a mineral the structure of the mineral may affect the light in various ways: e. g. the light's colour may be altered and its intensity may be reduced or blocked entirely. These differing effects are useful for identifying minerals and for delineating individual crystals and sometimes for showing internal structures that cannot be seen with normal light. Petrological microscopes used by geologists have the dual capability of viewing rock thin sections with normal light AND polarised light and comparing the different images is invaluable in mineral identification. They also have stages which rotate allowing crystals to be viewed in different orientations for yet more information.
The illustrations in this website are side-by-side images of rock thin sections photographed by the author using a petrological microscope at the University of Regina (Courtesy of Dr. Donald M. Kent). The left image is in normal light, the right in polarised light. They show a variety of minerals and structures from a miscellaneous collection of rocks and the author hopes that visitors find them interesting and educational.
The digital images displayed here were prepared for viewing on a computer screen set for 1024 x 768 pixels. As the top of each image shows approximately 0.75 mm of the original thin section the magnification on the screen is just about 200 times. To view the images either go to the Image Index and click on the text links or go to the Thumbnail Pages, of which there are two, and click on the thumbnail images.
The original images are on 35 mm photographic slides and were prepared by the author for a lecture to the Prairie Rock and Gem Society in Regina. They were presented side by side using two projectors and two large screens giving an ultimate magnification of 2500 times.
The author retains copyright of all website material but allows fair copying for educational use.
Colin Paterson, owner and webmaster of mediacooks.com, has the author’s grateful thanks for advice, assistance and webspace.
About the Author
Dr. Douglas F. (Doug) Paterson was born and educated in Scotland. He was a Geologist with the Geological Survey of Saskatchewan in Regina from 1965 through 1998 during which time he also gave occasional lectures at University of Regina and to other interested organisations and groups. Contact him at firstname.lastname@example.org