Light can be polarized in two ways. If a shiny surface such as water or an automobile window reflects light, it can be partially polarized. Or, you can use a polarizing filter that passes only the light that has the electric field in one orientation. A Polaroid® filter has long, thin molecules, all oriented in the same direction, embedded in plastic. The molecules absorb light that has an electric field parallel to the long dimension of the molecules. Therefore the light passing through the Polaroid® has an electric field perpendicular to the long dimension of the molecules.