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Light is almost popularly known to bend when passing from one transparent material to another at any angle other than perpendicular to the boundary of the two materials. This is called the index of refraction or a measure of how effective a material is in bending light coming from a vacuum. This is simply referred to as index n. This index n can be expressed as a relationship between the velocity of the light in a vacuum and the velocity of light in the material. The index of refraction of a vacuum is usually 1.0 and for all other materials n is greater than 1.0. The indices of almost all minerals fall in the range 1.4 to about 2.0. For optical mineralogy purposes, velocity in air could be considered as 1.0 because it is almost the same as the velocity in a vacuum. It is also important to note that a high index of refraction indicates a low velocity and vice versa.


            Snell’s Law is the equation that allows the calculation of how much the light will be bent on traveling from one material to another. From this equation, it can be found whether the light passes from material 1 to material 2 or the other way around. Generally speaking, a light is usually refracted away from the normal on entering a material with lower refractive index. It is also usually refracted towards the normal to the boundary on entering a material with higher refractive index.


            A light must be bent when entering a material with different index. The light in first material would strike the boundary at a certain angle. When entering the next material, the light usually slowed down. Because the Frequency does not change, it can be said that the wavelength of the light in the second material must be shorter than its wavelength when it passes through the first material. This is so because the velocity in material 2 is slower than in material 1. For the wave fronts to be closer together in the second material, they should be bent at the boundary. For the light to bent in entering thru second material, the wave normals must be also bent. These wave normals are found perpendicular to the wave front and they usually indicate the direction to which the waves are moving.


            Snell’s Law can be applied to both anisotropic and isotropic minerals. The only difference in anisotropic materials is that the angles produced must be measured through the wave normals and not through the rays. This is so because usually in anisotropic minerals the rays and the wave normals may not be found coincident.

Wednesday, February 27th, 2008 at 3:48 am
Optical Mineralogy
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