Polarization
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.