Conoscopic interference pattern: Difference between revisions

Revision as of 01:45, 10 January 2016

Revision as of 01:46, 10 January 2016

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:''This page is about the geology/optical mineralogy term. For general information about interference, see [[Interference (wave propagation)]] or [[Interference pattern]]s''.

A '''conoscopic interference pattern''' or '''interference figure''' is a pattern of [[Birefringence|birefringent]] colours crossed by dark bands (or ''isogyres''), which can be produced using a [[Geology|geological]] [[petrographic microscope]] for the purposes of mineral identification and investigation of [[Optical mineralogy|mineral optical and chemical properties]]. The figures are produced by optical interference when diverging light rays travel through aan optically non-isotropic substance - that is, one in which the substance's [[refractive index]] varies in different directions within it. The figure can be thought of as a "map" of how the birefringence of a mineral would vary with viewing angle away from perpendicular to the slide, where the central colour is the birefringence seen looking straight down, and the colours further from the centre equivalent to viewing the mineral at ever increasing angles from perpendicular. The dark bands correspond to positions where optical extinction (apparent isotropy) would be seen. In other words, the interference figure presents all possible birefringence colours for the mineral at once.

Viewing the interference figure is a foolproof way to determine if a [[mineral]] is optically uniaxial or biaxial. If the figure is aligned correctly, use of a [[sensitive tint plate]] in conjunction with the microscope allows the user to determine mineral ''optic sign'' and ''optic angle''.