I have what's called a polarizing filter for my camera. The camera part is is new to me, so I'm going to describe it in really basic terms. The science of it I kind of get. Polarization for light means that the light is filtered out so that only beams going perpendicular to the filter are allowed to pass and all beams that hit the filter from another direction are reflected. This makes it sound like it's an on/off thing, when it's not exactly like that with the camera filter. Physically, the circular polarizing filter is a round piece of glass that screws on the end of the lens. I'm talking about the 18-55mm zoom lens on my Digital Rebel XT, ymmv. So this dark plate of glass has a metal ring around it and it screws on the end of the lens. The filter can actually turn to adjust the amount of polarization it does. So you see that it never perfectly polarizes the light. The stronger the polarization, the more extra rays of light are being deflected and hence the less light there is in the picture. This can be okay outdoors, but the effect is very strong indoors. I decided to take a couple quick pictures for comparison. One has the filter turned to allow in as much stray light as possible, the other has basically the same shot with the filter set to let in as little as possible. The photos are just some coins (Canadian, American and Euro) lying on my dresser. The wood finish casts harsh reflections in the first picture, but in the second you can see that these reflections are greatly reduced. The side effect of reducing the available light is that I also had to set the shutter speed a little slower for the polarized shot.