There are two types of Polariser, circular and linear. 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.
Ideal polariser for autofocus SLR cameras. Slots into the Filter Holder which is then rotated. Reduces exposure 1⅔ stops when fully polarised.
When light from the sun bounces off a flat non metallic surface, such as glass or water, it becomes polarised with all of the reflected light waves vibrating in the same plane. This reflected glare can be removed by the polarising filter. On a clear 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 90 degrees from the sun - any white clouds will stand out impressively.
If used alone, the polariser can be placed into the filter holder and the holder rotated until the desired effect is achieved. If used with a grad filter, the grad should be positioned first, then the polariser added – you will have no rotation possible without turning the grad, but will have a choice of two axis of polarisation, achievable by turning the filter through 90 degrees.