Conoscopic interference pattern: Difference between revisions

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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 a 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''.

== Creation of a figure ==

In [[optical mineralogy]], a [[petrographic microscope]] and cross-[[polarized light]] are often used to view the interference pattern. The microscope's condenser is brought up close underneath the specimen to produce a wide divergence of polarized rays through a small point. This is done by placing a [[Bertrand lens]] (Emile Bertrand, 1878) between a high-power microscope objective and the eyepiece. There are many other techniques used to observe the interference pattern.

In [[optical mineralogy]], a [[petrographic microscope]] and cross-[[polarised light]] are often used to view the interference pattern. The thin section containing the mineral to be investigated is placed on the microscope stage, above one linear polariser, but with a second (the analyser) between the objective lens and the eyepiece. The microscope's condenser is brought up close underneath the specimen to produce a wide divergence of polarised rays through a small point, and light intensity increased as much as possible (e.g., turning up the bulb and opening the diaphragm). A high power objective lens is typically used. This both maximises the solid angle subtended by the lens, and hence the angular variation of the light intercepted, and also increases the likelihood that only a single crystal will be viewed at any given time.

To view the figure, the light rays leaving the microscope must emerge in parallel. This is typically achieved either by pulling out the eyepiece altogether (if possible), or by placing a [[Bertrand lens]] (Emile Bertrand, 1878) between the objective lens and the eyepiece.

Other techniques may also be used to observe the interference pattern.

== Characteristic figures of uniaxial and biaxial minerals ==

A uniaxial mineral will show a typical 'Maltese' cross shape and its isogyres, which will revolve/orbit around a projection of the optical axis as the stage is rotated.