8.8 Filters and Polarization
The part of the electromagnetic spectrum that is visible to the human eye is called visible light, or often just light. It actually covers a very small range of waves, with their wavelengths ranging from about 350 nanometers for violet to about 650 nanometers for red. However, white light which we see everyday is not in the spectrum as a single wavelength. As you may already know, this is because white light is made up of many different colors of light mixed together. This can be seen when a white beam is shone into a prism, which splits the white light into its constituent hues, or colors.
If you have ever been to a disco, you will know there is another way of creating light beams of different colors, without the use of a prism. This is done with filters. A filter is a material that only allows through it the color of light that is its name. For instance, a red filter will only let red light through it. All other wavelengths are absorbed by it. Because yellow light is actually a mixture of two wavelengths, red and green, a yellow filter will let both red waves and green waves through it. Here is a diagram of the three primary colors of light, and their secondary and tertiary colors.
You can see in it that yellow light is made up of green and red.
Suppose a very creative disco owner wants to make a new color light beam. To his white disco light he attaches first a red filter, then a green filter. What will he see when he turns on the light? Nothing at all! This is because the red filter only lets through red light, which, when it encounters the green filter, (because it is not green) is absorbed.
Polarizers are another way of changing light rays. They are effectively filters that separate electromagnetic waves in a different way. An electromagnetic wave, as its name hints, is actually made up of two wave components: an electric wave and a magnetic wave at right angles to each other. These waves induce each other, meaning the electric wave induces an oscillating magnetic field, which propagates as (induces) a magnetic wave, which induces an electric field . . . and thus coexist and are propagated together.
In the diagram above, the red rectangle indicates the electric field, while the blue one is the magnetic field. Waves have been filled in both to illustrate their coexistence.
However, any given electromagnetic wave is not composed of only one electric component and only one magnetic component. In fact, there are several electric fields, and therefore several magnetic fields. Here is a model for the cross-section of a ray of electromagnetic light, showing only the electric field planes. (magnetic field planes simply exist perpendicular to these lines)
For a polarizer, you can imagine an opaque sheet with a slit that blocks either all the electric or all the magnetic fields, but one.
The diagram above demonstrates this. If this polarizer were moved in front of the electromagnetic wave, the only electric plane that would be able to get through would be the one diagonal one going from the top left to bottom right. Light coming out of a polarizer has only one plane for an electric field, and one plane for a magnetic field.
What happens when two polarizers are placed in front of a light source? Almost the same thing as when two incompatible filters are placed in front of it: no light gets through. However, due to a few incongruities, two polarizers become totally opaque only if they are placed at right angles to each other (i.e. the "slits" are at right angles).