# Optic axis of a calcite crystal

1. Sep 23, 2011

### prashant07

What is an optic axis of a calcite crystal ?

2. Sep 23, 2011

### chrisbaird

Calcite is a birefringent crystal, meaning that light that strikes and goes through the crystal will refract differently depending on its polarization. This is because the crystal's atomic structure is different in one direction than in another direction. If you shine a light with mixed polarization through such a crystal (such as sunlight bouncing off of a pencil tip), then the two different polarizations in the light will respond differently to the crystal. One polarization will bend more than the other, so that out the other side you will get two beams of light appearing at different places (e.g. leading to two images of a pencil tip when you look at it through the crystal. If you send in polarized light (like from a laser), it will not split because it can only refract in one way.

The optic axis is the direction in the crystal where if you shine the mixed-polarization light in at this direction, it will not split into two beams. In this direction, the atomic structure looks the same in both directions.

As an oversimplified picture, consider the atomic lattice to be a simple rectangular grid with the atoms closer in one dimension than the other:

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A beam of light going into the screen will see a difference in this structure from side-to-side versus up and down. the part of the beam with the electric field up and down with react differently than the part of the beam with electric field going side-to-side in this picture. If instead, the light beam came from the right and headed left across the screen an encountered this lattice, and if the atoms where spaced into the screen with the same spacing as they are up and down, then the beam would see a uniform grid. This would be the optic axis.

You can get a piece of calcite inexpensively at most science museum gift shops or mineral stores, and then do quick experiments orienting it different ways and looking at how the double image is affected to get a feel for this.

3. Sep 24, 2011