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Electric and magnetic vectors, which vibrate at right angles to the direction of in which the radiation is moving are said to be constituted in all electromagnetic radiation including light. It is necessary to consider only vibration of the electric vector for mineral optics purposes. This is so because the behavior of the light is greatly affected by the interaction of the electric vector with the electrical character of the atoms and the chemical bonds in minerals. For the purpose of mineral optics, the forces arising from the magnetic vectors of light can be ignored because they are generally very small and insignificant. However, it is very important to note that the vibration direction of the electric vector is transverse. It vibrates perpendicular to the direction in which the light wave is propagating. In some ways, the vibration direction of the electric vector is analogous to the movement of water in a water wave or the movement of the solid earth with the passage of an earthquake S wave. The energy is usually propagated through the material in both cases. But the particles of water or earth are caused to move from side to side as the wave passes. However, the analogy in not complete because with light it is not a matter that vibrates from side to side but rather an electric field, which oscillates from side to side direction. When speaking of the light’s vibration direction, it refers to the electric vector vibration.


            The same nomenclature applied to any wave phenomenon can be used to describe a light wave. A light wave has velocity, frequency, and wavelength. Wavelength is defined as the distance from one wave crest to another. The relationship between the three variables can be expressed in equation Frequency = Velocity/wavelength. Frequency, therefore, is defined as the number of wave crests per second that pass a particular point. It is usually found expressed in Hz or cycles per second. The Frequency of light remains constant regardless of the material that the light travels through. But this is of course with some exceptions, which do not affect it like fluorescence for example. It should be understood that if the velocity changes, the wavelength also must change. Consider for example the wave train that passes through a piece of glass. The wave train is slowed down and the number of wave crests that enter the glass per second is the same as the number of wave crests that exits the glass. Thus, the Frequency remains constant because the number of wave crests that pass a point inside the glass per second is the same as outside the glass. However, the waves bunch up resulting to shorter wavelength because the velocity in the glass is substantially slower than in the air.


            The light passing through a mineral or through space is considered to be composed of innumerable waves traveling together rather than consist of a single wave. A wave en masse is therefore convenient to consider. A surface that connects similar points on adjacent waves is called a wave front. A wave normal on the other hand is a line constructed at right angles to the wave front. A wave normal also represents the direction that the wave is moving. The direction of propagation of light energy is called a light ray. Materials having light velocity that are the same in all directions are isotropic. In this kind of material, the light ray and wave normal coincide. Anisotropic materials on the other hand have light velocity different in different directions. The wave normal and the light ray directions in anisotropic materials are usually not parallel.

Wednesday, February 27th, 2008 at 3:47 am
Optical Mineralogy
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