Polarization of light is useful, after it has been filtered. A filter will separate light with one type of polarization from other types. Most daylight, or light from a light bulb, has a mix of polarizations (lasers are an exception). The filter works much like trying to put a playing card through a comb - only if the card is turned the right direction will it fit. Light that is turned a different way will be blocked by the filter. Liquid crystal displays (LCDs) use this to block light to make letters or numbers on a display. Glasses that have different polarization filters for each eye can separate light meant for the left eye and for the right eye. This is a common way to make 3D movies and 3D television.