What is Filter?
[[File:Kaffeefilter.jpg|thumb|A [[coffee filter]] acts like a [[sieve]]]] A '''filter''' is a [[device]] used to remove unwanted parts from something. For example, removing solid [[particles]] from a [[liquid]]. Filter can also mean the act of filtering: it can be used as a [[verb]]. The different branches of [[science]] and [[technology]] often mean a particular kind of device when ''filter'' is mentioned. ==In chemistry== [[File:Water filter contents.jpg|thumb|The contents of a domestic water filter. The black objects are [[activated carbon]]. The white objects are [[ion-exchange resin]].]] The simplest filter is the kind called [[filter paper]]. Liquids can pass through the paper. Solid particles are stopped. More advanced filters are possible. Two that are often used to make water more pure are [[activated carbon]] and [[zeolite]]s. Filters are used to remove unwanted items and/or impurities in the substance you are filtering. ==In electrical science== {{Main|Filter (electronics)|}} The most usual thing meant by filter in [[electricity|electrical science]] is a wave filter. One common kind lets through some [[frequency|frequencies]] and stops all others. This is a band pass filter. For instance, in a [[radio]] it can select the [[radio station]] the listener wants to hear. The frequency of that station is passed, but all others are rejected. There are many other uses, including use in [[telephone]]s and [[audio]], see: [[audio crossover]]. There are [[digital]] filters which can be used to filter things besides frequencies. A common use is to improve the quality of pictures. For instance, there are filters which can make the edges of objects look more clear. ==In optics and photography== [[Optical filter]]s act on [[light]]. They change it certain ways. This can be used for [[photographic filter]]s. ==In mechanics== Filters can be made from mechanical components. They will do very similar things to electric filters. Most especially, mechanical filters can be wave filters. That is, they c