Hoya 77mm HD2 Circular Polarizer Filter - 8-layer Multi-Coated Glass Filter
Light rays which are reflected by any surface become polarised and polarising filters are used to select which light rays enter your camara lens. They allow you to remove unwanted reflections from non-metallic surfaces such as water, glass etc. They also saturate colors and increase contrast. Polarizers come in two variations but have the same effect. Linear polarizers work with older camera meters; Circurlar polarizers are designed for today's newer split-beam metering systems used in auto focus cameras. Check your camera's manual.