A polarizer changes electromagnetic energy, such as visisble light, from a mixed, or unpolarized beam into a single polarized beam. Many optical instruments like cameras, telescopes, and microscopes use this technology as either integrated or screw-on devices to view specific types of light. There are two general kinds of polarizers: absorptive and beam splitting.
An absorptive polarizer filters out the unwanted beams by absorbing them, and leaves behind just the desirable ones. The most common type of absorptive filter is a wire grid, which allows only a single type of beam to pass through. Polaroid™is one of the most popular name brands of absorptive polarizer, as it uses stretched polyvinyl alcohol polymer chains to filter light. The famous, but now-obsolete, instant picture film used the technology, and it is still in use as a film for sunglasses, liquid crystal displays, and microscopes.
A beam-splitting polarizer does just what it's name says, in that it splits a beam into two opposing polarizations. Much like a magnet has a positive and negative end, so does a beam of light, although the difference is not quite so easily understood. The polarization of light through beam splitting will generally produce one pure beam and one mixed beam, rather than two pure beams.
The most commonplace usage of a polarizer is in photography. A lens attachment reduces reflections, and increases the saturation of color. The contrast between clouds and sky is more prominent, and details like leaves tend to look crisper when using a polarizer. A polarizer is most effective when shooting at an angle 90% from the sun. It is not effective for a photographer to shoot with the sun at his back.
Astronomers use polarizing filters with their telescope eyepieces to focus on a celestial object. The filter reduces glare without altering the true color of the object being viewed. This glare reduction allows for a clearer view of the object, and the ability to see more terrain details and anomalies.
Microscopes also use polarizers to study various materials. A polarizing microscope uses two types of filters, a polarizer situated beneath the specimen, and an analyzer situated above. With the specimen between the two, a light free environment is made possible. The analyzer can be moved into or out the field of view to give an observer various levels of polarization. The technology permits the viewing of either reflected or transmitted light. Reflected polarized light is particularly useful for studying mineral oxides and sulfides, silicon wafers, and metals.