Polarising filters are used to manage light at wavelength scale, and if magnified could be likened to a series of microscopic slats. In use, as the filter is rotated, it either transmits or blocks the waves of light that are vibrating in particular orientations.
When light from the sun bounces of a flat non metallic surface, such as glass or water it becomes polarised - ie: all of the reflected light waves vibrate in the same plane. This reflected glare can be removed by the polarising filter as it is rotated to the optimum position. In photographic terms this can render water or glass transparent, and gives the impression of saturating colours in a scene because much of the reflected glare from the subject is removed by the filter.
On a sunny day, much of the light in the sky is also polarised, and the filter will give a very strong blue effect when used at an angle close to 90 degrees to the sun – any white clouds will stand out impressively against such a dark blue background.
There are two types of polariser, linear and circular. These names bear no relation to the physical shape of the filter itself but refer instead to the way the light waves are transmitted through the filter to achieve the polarising effect. Circular Polarisers spin the light waves so as not to confuse the internal optics of modern reflex cameras whereas linear polarisers work with the simpler optical systems in more traditional cameras. If your camera requires a circular type polariser then it will normally be mentioned in the user manual.
Screw-in polarisers are attached to the filter system via a 105mm Accessory Ring. The Accessory Ring fits onto the front of the holder and allows the polariser to be rotated independently of any graduated filters that may also be used in the holder.