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Polarizers

Introduction

Unpolarized electromagnetic radiation has electric field vectors in all directions perpendicular to the direction of light propagation. Polarizers create linearly polarized light by selective absorption, reflection, or refraction of the two orthogonal polarizations of unpolarized light. Circularly and elliptically polarized light are obtained by introducing a phase shift to linearly polarized light with a retardation plate.

Polarization by absorption: Polaroid is a plastic film that has all of its long organic molecules oriented in the same direction. These molecules absorb radiation of one polarization, thus transmitting the orthogonal polarization.

Polarization by reflection: The reflection of light from a surface is polarization and angle dependent. At Brewster's angle, which is material and wavelength dependent, the relectance of p-polarized light becomes zero, and the reflected light is completely s-polarized. Modern polarizers use dielectric coatings to achieve very high (99%) reflectance and transmittance to separate the two polarizations of light.

Polarization by refraction: Birefringent crystals, calcite is an example, have different indices of refraction for different polarizations of light. The indices are called the ordinary index of refraction, no, and the extraordinary index of refraction ne. With the exception of the optic axis, which is the one direction in the crystal at which no and ne are equal, two rays of orthogonal polarizations entering the crystal will be refracted at different angles and therefore separated spatially on leaving the crystal.

Creating circularly polarized light: Linearly polarized light is the superposition of two orthogonal polarizations that are in phase. Phase shifting one of these polarizations by 1/4 of the wavelength (90o) creates circularly polarized light. Circularly polarized light is therefore obtained by linearly polarizing a light beam, and retarding the velocity of one polarization by passing it through a quarter-wave plate. The quarter-wave plate is a birefringent crystal of a thickness such that the two indices of refraction produce the correct phase shift for a given wavelength of light.


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