WAVE FRONT, WAVE NORMAL
With an infinite number of waves travelling together from a light source, we now define:
- Wave front - parallel surface connecting similaror equivalent points on adjacent waves.
- Wave Normal - a line perpendicular to the wavefront, representing the direction the wave is moving.
- Light Ray is the direction of propagation of the light energy.
Minerals can be subdivided, based on the interaction of the light ray travelling through the mineral and the nature of the chemical bonds holding the mineral together, into two classes:
Light waves travelling along the same path in the same plane will interfere with each other.
Isotropic materials show the same velocity of light in all directions because the chemical bonds holding the minerals together are the same in all directions, so light travels at the same velocity in all directions.
In isotropic materials the Wave Normal and Light Ray are parallel.
- Examples of isotropic material are volcanic glass and isometric minerals (cubic) Fluorite, Garnet, Halite
Anisotropic minerals have a different velocity for light, depending on the direction the light is travelling through the mineral. The chemical bonds holding the mineral together will differ depending on the direction the light ray travels through the mineral.
In anisotropic minerals the Wave Normal and Light Ray are not parallel.
- Anisotropic minerals belong to tetragonal, hexagonal, orthorhombic, monoclinic and triclinic systems.