Light is transverse electromagnetic wave. Transverse means that the oscillation of energy is perpendicular to the direction of light.
Mathematically, there can be unlimited amount of directions in a space that are all perpendicular to a given line. In reality, this is the case for most light rays. The direction of energy oscillation for the light ray is a mixture of all the perpendicular directions.
Unless the light is polarized. Polarization refers to removing of all the light energy from certain directions of oscillation but one. It happens in nature too: reflection from a water surface is polarized, and certain part of sky, depending on where the sun is. That makes polarized lens very useful in photography. The energy, i.e. intensity of light in certain part of the picture is changed by applying the lens.
To make the polarized lens or sheet, usually a coating is put on a film, and the film is stretched. The stretching gives the film as well as the coating a different structure. Because of the unidirectional treatment of the material, it absorbs some of the light energy and let only the direction of light that aligns to the lens/sheet's direction to through, hence you have polarized light after the lens/sheet.
If the lens/sheet is made perfect, only one direction of oscillation of energy can go through it. So if you apply another one of those, at a 90 degree angle to the first one, all the light the goes through the first one should be blocked by the second one. This configuration can tell you if those polarization lens/sheet have defect, because defects allow light to go through both layers.
It is very easily demonstrated by two pairs of polarized sun glasses or 3-D movie glasses. If you put two lenses on top of each other, and rotate only one of them, you can see that certain direction will allow most light to go through, and at 90 degree to this direction, no light can pass both lenses.
In summary, polarized light is the kind of light that energy oscillates at certain direction, not others. Polarized sheets are made to absorb all light but one direction of energy oscillation, hence makes polarized light. When a 90 degree oriented polarized light shines on a polarized sheet, no light shall pass, unless there is a defect on the sheet.
Hope this helps,
Heng
