Polarization of Light

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