A Pol filer (Polarising/Polarizing filter) is probably the one filter everyone should own. A pol filter will filter light waves travelling in a specific direction. To understand how it works you need to understand how light works. This is a long and complicated topic so I will only skim over the basics (any one who wants to know more should read ” Light – Science and Magic. An Introduction to Photographic Lighting” from Focal press ISBN-10: 0240808193 ISBN-13: 978-0240808192)
OK I don’t know how to emplane this so I’m just going to type and see how it comes out. Sorry if its all confusing and such.
Important notes about light
Light is made up of lots of tiny waves. These waves have different frequencies that can be seen as different colours. Mixing the different frequencies will result in different colours, just like mixing paint. But unlike paint when you mix all the visible frequencies together you will get white and not black. This is important to know as when you use a filter you and adjusting those tiny waves in one way out an other. Light waves bounce of different colours and surfaces differently, some surfaces/colours reflect light well and others don’t. Its these reflected light waves that are generally interesting when using a Pol filter.
When light hits the lens opening it may be coming from all directions, creating reflections or highlights that are unwanted. Unlike the ND filters that simply darken down an exposure a pol filter will only allow light waves that are travelling in a specified direction through the filter.
As you can see the waves of unpolarised light (coloured blue for this example, on the left) are travelling at 90 degrees to the other waves. A pol filter will filter light waves travelling in a specific direction there for the waves travelling in the “wrong” direction are filtered out. If you turned the filter 90 degrees it would only allow the light waves through that are blocked in this example.