Colorful rocks in transparent water at the coastline of Western Norway (Sund, Golten) viewed through a polarizer to take away the reflections from the water surface.
Nikon D3X, Nikkor 24-70mm 2.8G at 24mm, f/11, 1/20sec and ISO200 and a circular polarizer.
The polarizing filter used with most modern cameras is a circular polarizer. The first stage of the polarizer is a linear filter which filters out light that is linearly polarized in a specific direction. The second stage, for technical reasons related to the auto sensors within the camera, then circularly polarizes the light before it enters the camera. The polarizing filter has two applications in both color photography and black-and-white photography: it reduces reflections from some surfaces, and it can darken the sky. Light reflected from a non-metallic surface becomes polarized; this effect is maximum at Brewster’s angle, about 56° from the vertical (light reflected from metal is not polarized, due to the electromagnetic nature of light). A polarizer rotated to pass only light polarized in the direction perpendicular to the reflected light will absorb much of it. This absorption allows glare reflected from, for example, a body of water or a road to be much reduced. The benefits of polarizing filters are largely unaffected by the move to digital photography: while software post-processing can simulate many other types of filter, a photograph does not record the degree of polarization, so the optical effects of controlling polarization at the time of exposure cannot be replicated in software. (Wikipedia)