According to the wave theory of light, ordinary light is composed of electromagnetic waves vibrating in all possible planes through the line of propagation. Plane polarized light vibrates in a single plane called the plane of polarization. Most of the optical properties of minerals observed in thin-section depend on a light source that is plane polarized.

POLARIZED LIGHT and CRYSTALS

A light wave is an oscillating electric field (having both electrical and magnetic properties), possessing alternating + and - charges. Electrons in crystals, especially valence electrons, will respond to the changing electric field associated with light waves. In crystals acted upon by a changing electric field (light waves), the density of its orbiting electrons is greatest in the direction of greatest electrical attrating and the atom is said to be polarized. The degree of polarization depends on the degree that the outer electrons are bound to one another an dhow free they are to vibrate due to external forces.

The pattern of polarization depends on the nature of the electron's closest neighbor where a dipole can be developed. The pattern of polarization is thus strongly tied to the crystal structure of minerals. the polarizaiton of atoms in a isotropic medium is either statistically random (amorphous substances) or directionally uniform (isometric crystals), and the atoms are not consistently polarized by their atomic neighbors. In anisotropic minerals (tetragonal, hexagonal, orthorhimbic, monoclinic, and triclinic crystal systems), atoms experience non-uniform electrical attraction from dissimilar neighboring atoms to become polarized in the direction of strongest attraction. The plane of maximum polarization is usually the plane of maximum atom density, and the plane of minimum polarization is perpendicular to it. The atoms of chain (Inosilicates) or sheet (Phyllosilicates) mineral structures are most strongly polarized in the plane of the sheet or direction of the chain.

Polarization strongly effects the velocity of light waves. The stronger the polarization, the slower the velocity of light and the greater the refractive index. Depending on th edegree of polarization in the crystal, each light wave will be transmitted with a different velocity, and will have a different refractive index. As example, in the structure of calcite (CaCO₃) the carbonate ions are arranged perpendicular to one cube diagonal, which becomes the unique or c- crystallographic axis. The ions are held tightly together by strong, largely covalent bonds that are much stronger than any external bond and are highly polarized in the plane of the atom. Light waves transmitted parallel to the c-crystallographic axis vibrate parallel to the planes of the carbonate ions, increasing polarization already present. Calcite therefore shows a relatively high index of refraction and a corresponding low velocity for waves advancing parallel to c, regardless of their vibrational direction normal to the axis. Light waves transmitted through the calcite crystal perpendicular to the c- crystallographic axis advance with different velocities depending upon their vibrational direction. Waves vibrating perpendicular to c also vibrate in the plane of the carbonate ions, increasing polarization and advancing with the same velocity as waves transmitted parallel to c. However, wave vibrating parallel to c vibrate at right angles to the plane of the carbonate ions, thus reducing polarization and advancing with greater velocity (index of refraction is lower).

Polarized light is obtained with a prism in which ordinary light (vibrating in all possible directions perpendicular to the propagation direction) is plane polarized, i.e., light vibrating in only one direction is transmitted.

Nicol prism-a polarizing prism made of calcite cut at a specific angle. Nicol prisms are extremely precise, but very expensive. Most petrographic microscopes are now equipped with the cheaper (but highly efficient) polarized sheets.

Polarizer-a polarizing prism located beneath the microscope stage (between the light source and the object of study). this restricts transmission of light to that vibrating in only one (N-S) direction. In effect, it plane polarizes the incident light beam.

Analyzer-a polarizing prism located above the microscope stage, between th esection and the eyepiece. This restricts the transmission of light vibrating perpendicular to the polarizer. The analyzer can be slipped in oro out of the light path. Light passing through the polarizer will not pass through the analyzer unless the vibration direction of the light is changed between the two prisms. Anisotropic minerals can perform this deed.

This page (http://www.ces.ncsu.edu/plymouth/programs/polar.html) created by
Vera MacConnell, Research Technician, I on November 11, 1997.
Last Updated on November 11, 1997.