Plane Polarized
Light PetroGlyph offers several optical microscope views of each thin section: Plane polarized light , cross polarized light, and reflected light. PetroGlyph also allows the user to view interference figures for each grain using the Bertrand lens function.
The plane polarized light image shows the thin section in light that has passed through one polar so all light rays vibrate in a single plane. Pleochroism is visible under plane polarized light. Pleochroic minerals have anisotropic crystal structures that absorb more light and/or different frequencies of light in some directions than in others, so the color of the mineral changes as the stage is rotated.
To view the plane polarized light image:
 | Select the Plane Polarized icon |
Cross polarized images, photographed with the polars at 15o increments, are available in PetroGlyph. Cross polarized light images are illuminated by light that has passed through the lower polar, the mineral, and then the upper polar. Because of anisotropy in the crystal structure, light that passes through anisotropic minerals is split into two rays that travel at different speeds and vibrate perpendicular to each other. Interference of these rays causes birefringence. Mineral extinction is a result of the interaction of the two rays with the polars. Anisotropic minerals become extinct every 90 degrees as the stage is rotated under crossed polars, when one of the vibration directions of light passing through the mineral is parallel to the lower polar. Isotropic minerals, such as garnet, appear black under crossed polars regardless of stage rotation because the minerals do not affect the light that travels through the lower polar; all light rays that pass through the isotropic mineral are cancelled out by the upper polar.
To view the cross polarized light image:
| Select the Cross Polarized icon |
To change the angle of stage rotation:
| Click on the arrows on the rotate polars icon |
Clicking on the arrows simulates turning the polarizer in clockwise or counterclockwise direction. This function simulates rotation of the stage of a petrographic microscope. The polarizer can be rotated in increments of 90 degrees (plane polarized light) and 15 degrees (cross polarized light).
Images of the thin section in plane polarized reflected light are also
available in PetroGlyph. In reflected light mode, light reflects off a mirror, travels
down through the objective, and reflects again off the thin section. Reflected light is
useful for identifying many opaque minerals, such as oxides and sulfides, which appear as
reflective, silvery or yellow grains. A reflected
light image of a garnet mica schist is shown below.
To view the reflected light image of a thin section:
| Select the Reflected Light icon |
The Bertrand lens is used to view interference figures. Using the Bertrand lens, you can determine if a mineral is uniaxial orbiaxial. Interference figures for uniaxial minerals have isogyres that form a cross that rotates in a circle as the stage is rotated. In interference figures of biaxial minerals, the isogyres form two arcs (in some crystal orientations, the second arc may not be visible), which separate and come together as the stage is rotated. The Bertrand lens can also be used to estimate 2V, the angle between the two optic axes. Using the gypsum plate with the Bertrand lens, optic sign can be determined. Optic sign indicates which of the two rays of light traveling through the crystal has the higher index of refraction.
To observe optic axis figures:
| Select the Bertrand lens icon |
| Click on a grain |
PetroGlyph will display a short movie of the optic axis figure as it changes during rotation of the polars. (In this simulation, each grain does not have a unique movie; a near optic axis figure is shown instead.)
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