Paul Jolly Photography Chapter 18

Polarizing filters

Light when reflected contains light with a bias of polarization direction, thus we can use a polarising filter to selectively enhance or suppress reflective light sources: these sources can be from a pond, shiny surface or even the sky (reflected / refracted through water molecules in the air).
The two types of polarizing filter are linear and circular. The easiest one to understand is the linear filter. Light being reflected from the surface of water has a bias to be horizontally polarized; rotating the filter to align horizontal allows the maximum reflected light. Rotating the filter to vertical lessens the reflected light allowing the camera to 'see' into the water.

In a similar way the sky in a photograph can be enhanced (as shown on the right of these two photographs).
Polarizing filters can only be used when taking the photograph as polarization information is lost with current camera sensors, thus post processing cannot achieve exactly the same effect.