Process

Exploring Rocks: Polarized Light

THIN SECTIONS AND LIGHT
The photomicrographs in Splendor in Stone are views through a petrographic microscope of ultrathin slices of rock, called thin sections. In most of the photomicrographs, the thin sections are between two polarizing filters called the polarizer and the analyzer. The light interacts with these filters and the minerals in thin section to create the colors you see.

WHAT IS POLARIZED LIGHT?
Ordinary light can be thought of as rays traveling in straight lines, pulsating in all the directions at right angles to their paths of travel. Polarizing filters allow only light vibrating in one direction to pass through. Such light is plane polarized; each ray travels and vibrates in a single plane. If a second polarizer, the analyzer, is placed in the light path with its plane of polarization at right angles to that of the first, the light emerging from the first polarizer will be completely blocked by the second, and no light will pass through. This arrangement is called crossed polarizers. At any other angle some light will get through.

However . . .

MOST MINERALS POLARIZE LIGHT TOO
Light interacts with the atomic structure of the minerals as it passes through them. When light enters mineral grains, it slows down.

Isotropic minerals have no other effect, and the light from the polarizer passes through unchanged. Garnet is one of this type; therefore, the round grains of garnet are black under crossed polarizers.

Most minerals split polarized light into two new separate polarized rays with their planes of polarization at right angles to each other. The rays travel through the mineral at different speeds and when they emerge, one is ahead of the other. When these two rays strike the analyzer, it resolves their two planes into one and they interfere with each other. This produces new colors called interference colors, shown by the bright colors in the mica and the grays of the quartz. The shades of gray show separate grains of quartz with different orientations.

A specially ground slice of gypsum, called a gypsum plate, is added to the light path of the microscope to enhance the colors. The garnet takes on the interference color of the gypsum plate.

 


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PLANE POLARIZED LIGHT
Garnet - Mica Schist
Dutchess County, New York

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CROSSED POLARIZERS
Garnet - Mica Schist
Dutchess County, New York

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CROSSED POLARIZERS WITH GYPSUM PLATE
Garnet - Mica Schist
Dutchess County, New York

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