Atlas of Optical Crystallography How to play with polarized light Olaf Medenbach Institut für Geologie, Mineralogie und Geophysik Ruhr-Universität Bochum 44780 Bochum, Germany olaf.medenbach@rub.de © by Olaf Medenbach

Useful polaroids as analyzers are available: A perfect light source for your experiments in polarized light is the TFT flat screen of your computer or even your cell phone. Useful polaroids as analyzers are available: From the camera shop next door. Please note that these are usually laminated with a quarter wave foil on one side to provide circular polarized light for the correct function of the lightmeter of the camera. This side must point towards your eye – otherwise you will not get extinction with crossed polarizers. quartz twin  c muscovite

A perfect light source for your experiments in polarized light is the TFT flat screen of your computer or even your cell phone. Useful polaroids as analyzers are available: as sunglasses or special glasses for fishermen. Fishermen use them to reduce the reflections from the surface of the water which are more or less perfectly polarized (cf. „Brewster angle“ in the chapter on „Measurement of refractive indices”). You may have to readjust the glasses according to the vibration direction of your personal flat screen.

You will find that a lot of objects in your everyday life are birefringent, and that all basic observations in both, orthoscopic and conoscopic light can be made with these simple setups without any use of highly sophisticated optical instruments. Use your imagination and creativity; have fun and find out, that crystal optics is not an outmoded and complicated matter to bore students, but a beautiful and colorful living art and science. plastic containers show vivid interference colors

Scotch tape strips stacked on a glass plate to show crosses, stars or stairs show all effects of superposition of crystals such as addition and substraction of interference colors.

For conoscopic work bring the analyzer and the crystal under investigation as close to your eye as possible for maximum aperture, and turn and tilt the crystal. Suitable are a lot of species which are easily available in appropriate sizes (minimum about 5 mm across), such as all micas, cleavage pieces of topaz, apophyllite, euclase, etc. A muscovite flake showing a retardation of first order red in orthoscopic light, and the direction of nZ marked on it with a pen works perfectly as a compensator.

A further advantage of the polarized light from a TFT flat screen is its spectral composition. Because it has only 3 narrow bands of wavelengths (red, green and blue), an almost infinite number of orders are visible rather than a „high order white“ beyond the eighth order. calcite c, 2mm thick