The amount of separation of the two rays is known as the BIREFRINGENCE. Click here* to see how this works as a practical matter. You'll see a piece of calcite placed over writing and the letters are DOUBLED! Most doubly refractive minerals don't exhibit this characteristic as strongly as calcite but it is measurable and useful in identification.

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We can use the electromagnetic theory for light to explain how a light ray is split into two rays (FAST and SLOW) which vibrate at right angles to each other.

The above image shows a hypothetical anisotropic mineral in which the atoms of the mineral are:

1. closely packed along the X axis
2. moderately packed along Y axis
3. widely packed along Z axis

The strength of the electric field produced by the electrons around each atom must therefore be a maximum, intermediate and minimum value along X, Y and Z axes respectively, as shown in the following image.

With a random wavefront the strength of the electric field, generated by the mineral, must have a minimum in one direction and a maximum at right angles to that.

Result is that the electronic field strengths within the plane of the wavefront define an ellipse whose axes are;

1. at 90° to each other,
2. represent maximum and minimum field strengths, and
3. correspond to the vibration directions of the two resulting rays.

The two rays encounter different electric configurations therefore their velocities and indices of refraction must be different.

There will always be one or two planes through any anisotropic material which show uniform electron configurations, resulting in the electric field strengths plotting as a circle rather than an ellipse.

Lines at right angles to this plane or planes are the optic axis (axes) representing the direction through the mineral along which light propagates without being split, i.e., the anisotropic mineral behaves as if it were an isotropic mineral.