My Dad's 1952 Plymouth and 18-foot Mainliner trailer
From those Kodak pamphlets I learned the exposures to use for night scenes of the Jefferson and Lincoln memorials. I also learned that a polarizing filter can be used to darken the sky, or to reduce reflections from most surfaces.
The polarizing filter is still a magical tool, one of the few filters I still use. Digital photography has other ways of obtaining the effects once provided by the colored filters used for black and white photography, and the color correcting filters used with color slide film. But a polarizer is still always with me when I am photographing, since it can produce effects that no in-camera-processing or post-processing can achieve.
Polarized light is magical. Bees even use it to navigate, according to what I read. Normally light waves vibrate in all directions. But some of the blue light from the sky vibrates in only one direction - it has become polarized as it has reflected off of particles in the atmosphere. Or something like that. Light also becomes polarized when it reflects from a non-metallic surface. Why not from a metallic surface? I really don't know. But photographers can use this phenomena to bend light to their will.
A polarizer can prevent polarized light from reaching the sensor. Thus polarized skylight darkens to become a deeper blue, and glare from surfaces is pretty much eliminated. The glare reduction thing is pretty useful when photographing foliage in sunlight. Leaves look a richer green, and the color of flowers is deepened. In addition to darkening blue skies and cutting glare, polarizers can also reduce the effect of distant haze.
The glare-reducing effect can be striking in the fall. Here's a photo of the two maples that have grown to frame our garden gate. The sun is coming toward the camera from the left, and there is a lot of glare from the leaves:
Without polarizing filter
In the photo below I've added a polarizing filter, and rotated it until the color from the leaves appeared richest.
With polarizing filter
In addition to the deeper color of the leaves, the sky has also become darker. And there is another useful effect which is a bit more subtle. Notice that there is a bit more detail in the fence.If desired, there is enough detail there now that it could be enhanced a bit by post-processing. (Both of these photos are JPEGS straight out of my Nikon D300.) The reason for the increased shadow detail is that by cutting glare, the highlights in the scene have been darkened, and the overall contrast has been reduced, allowing an exposure that provides just a bit more light from those shadows.
If you don't have a polarizer, then get one. I use them even on point-and-shoots that don't mount filters by holding it in front of the lens, after first noting the orientation that gives the best effect. So get one and play with it. I'll bet you will have as much fun as I did with those clip-on sunglasses.
Some links for more reading about polarizers and polarized light: