Technical & Editorial
- Written by Scott Wilkinson
- Published on 15 November 2012
As you may know, light is a form of electromagnetic radiation that travels as a wave, somewhat akin to water waves. Unlike water waves, however, the crests and troughs of a light wave can be horizontal, vertical, or any angle in between. The angle formed by a light wave's crests and troughs with respect to horizontal and vertical is called its polarization.
Fig. 4: The polarization of most light is random, as shown in the upper part of this diagram. (Source: esemenyhorizont.uw.hu)
It's also important to know that the light from most sources does not have a single polarization. Instead, each photon—a "particle" of light—can have a different, random polarization, as shown in the upper portion of Figure 4.
A polarizer allows only light waves with a certain polarization—that is, waves with crests and troughs at a certain angle—to pass through them. For our purposes, a polarizer can be either horizontal or vertical, so only light with horizontal or vertical polarization gets through. Light waves with different polarizations are at least partially blocked; only the horizontal or vertical portion of each photon passes through the polarizer, depending on its orientation.
Normally, if light passes through a horizontal polarizer, it is then completely blocked by a subsequent vertical polarizer, as shown on the right in Figure 5. If you stacked a horizontal and vertical polarizer together and looked at a light source through them, you'd see only black, since no light would pass through both polarizers.
Most of us are familiar with polarized sunglasses, which allow vertically polarized light to pass while blocking horizontally polarized reflections, which cuts glare reflected from horizontal surfaces like the road and dashboard of the car. If you place the lenses of two such sunglasses next to each other and rotate one of them 90 degrees, you see virtually no light pass through both lenses.