Polarization of Light

When light is emitted from an object, it radiates out in all directions and planes. If we were to pass one of these rays through a special filter, we could change the orientation of the ray such that it only oscillated in one plane.

We call this process polarization and when we do it, we say the light is plane-polarized. If we then take a second filter that is in a different orientation than the first and place it in front of our polarized wave, the light will not be able to pass through.

(OK - I admit that these are pretty bad sketches. Check out one of the links below to see a better example.)

Natural light is unpolarized, although reflected light may become polarized. When light reflects off of a surface, it becomes polarized in the plane of the reflected surface. This happens on roads and water, for example. When we view this light though a polarizing filter, such as polarizing sunglasses, the reflected glare is eliminated. You can actually see very clearly through a river or lake with polarizing sunglasses on.

There is a formula to help us determine how much light will pass through polarizing filters:

where I is the intensity of the light as we see it, Io is the initial intensity, and q represents the angle between the
polarizing filters. For example, if we place two filters at a 45o angle to each other, we will get 1/2 of the original light out of the filters. If the filters are at 90o to each other, we will not get any light out (cos 90o = 0).

A special case of polarization occurs when an incident ray strikes a surface and the reflected ray is 100% polarized parallel to the surface. The reflected ray and the refracted ray are at 90o angles to each other. This is Brewster's Law. For more on this, try http://webphysics.davidson.edu/physlet_resources/bu_semester2/c27_brewster.html.

Check out these additional links to polarization:
http://www.physicsclassroom.com/Class/light/u12l1e.cfm

http://www.colorado.edu/physics/2000/applets/polarized.html

For Practice Problems, Try: Giancoli Multiple Choice Practice Questions (Questions 22-27)