In order to discover how polarization methods work, it is necessary to understand the difference between isotropic and anisotropic materials.
Refraction
The velocity of light depends on the nature of the material it travels through. When light enters any other material from a vacuum, it is slowed down. When light reaches a boundary between two materials some of it may enter the material and some of it will be reflected at the interface. The angle of reflection is equal to the angle of incidence. When some of the light enters the new material, it is refracted, or bent. The index of refraction of a material is a measure of how effective it is in bending light coming from a vacuum.
Courtesy of Peter Whitehead, James Cook University http://www.jcu.edu.au/~glpww/EA2001/Light/Light.html
In certain minerals, specifically those belonging to the isometric system,and in glasses, the chemical bonds are the same in all directions. These materials show the same velocity of light in all directions and the index of refraction is everywhere the same.
Courtesy of Peter Whitehead, James Cook University http://www.jcu.edu.au/~glpww/EA2001/Light/Light.html
In anisotropic materials, chemical bonding is not the same in all directions. Therefore, the velocity of light is different in different directions. Anisotropic minerals show double refraction. When light enters an anisotropic material, it is split into 2 rays. These rays are vibrating with different velocities and at right angles to each other. The rays have two different indices of refraction. Anisotropic minerals are further subdivided into uniaxial and biaxial minerals. Uniaxial minerals have one axis, the optic axis, along which light behaves as if the mineral were isotropic. Biaxial minerals have two axes along which light acts as if the mineral were isotropic. The behavior of light in biaxial minerals is slightly more complicated, so we will restrict our discussion here to uniaxial minerals. Both types exhibit double refraction.
When light enters a uniaxial mineral it is broken into two rays, the O and E rays. The ordinary ray, or the w ray, vibrates in the basal plane of the mineral, perpendicular to the major axis of the crystal, the optic axis. The "extraordinary" ray, or the e ray, vibrates in the plane that includes the optic axis.
Courtesy of Peter Whitehead, James Cook University http://www.jcu.edu.au/~glpww/EA2001/Light/Light.html
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