Yikes, how’d I let that get through? I’m really sorry! I intended the article to be less-technically focused and more of a focus on why they’re useful, but not to the point that I was disseminating incorrect information. I screwed up.

Thank you all very much for giving everyone the correct information!

@Dumitru, Dennis: Thank you, I’m glad you liked it!

@Alexander: Sure! You have my permission to reproduce it, in Russian, for a non-commercial blog. You have to maintain the links back to my site (goldengod.net) and author/photo credits though. :)

Article like this are sooooooooooooooooooooooooo tasty.

Light coming from a reflected surface is all the same wavelength. This allows the filter to eliminate reflected light on that specific wavelength, making the reflections fainter.”

This is not true. Polarization has nothing to do with wavelength. Please read this article and correct your own text:

http://en.wikipedia.org/wiki/Polarizer

Polarization of light has nothing to do with wavelength. If light reflected off a surface was all the same wavelength it would appear as a single rainbow color, not white or any other color.

Polarized light is light where the electromagnetic waves that make it up are oscillating (mostly) along a single plane. A light ray which undergoes reflection on a non-metallic surface becomes typically polarized along a plane parallel to the reflecting surface.

A polarizing filter is coated with a substance which filters out most waves that are not along a single plane of oscillation. By rotating the plane of the filter so that it is perpendicular to the plane of polarization of reflected light, you effectively block most of the latter.

No, same wavelength would mean it all had the same color, which it doesn’t. Reflected light does have roughly the same *polarization*, though. (See: http://en.wikipedia.org/wiki/Polarization )

Linear polarizers did this too, this isn’t limited to circular polarizers.

From what I understand, the “circular” refers not to the shape, or orientation of the waves, but to a little circular tweak the light gets on the way out of the filter on the way to the lens. I’ll return to this in a second, but first, the other problem with your explanation…

“Light coming from a reflected surface is all the same wavelength. This allows the filter to eliminate reflected light on that specific wavelength, making the reflections fainter. ”

No. Light which is all the same wavelength is all the same colour. What you mean is that “light coming from a reflected surface is all the same orientation.” Consider ripples of water on a pond: the water goes up and down, the distance between two “ups” is the wavelength. Tighter ripples are on blue, longer more relaxed ripples are red, intermediate length ripples are orange, yellow, or green. Now, as light waves travel through the air, they aren’t restricted to up/down movement like the surface of a pond; light waves can oscillate left to right, or at any other angle.

What a (linear) polarizing filter does is only let through light vibrating in one direction. As you turn it, you can select which direction that is. All wavelengths get through, but only if it is oriented correctly. Most light isn’t oriented correctly and is mostly filtered out, resulting in about a two-stop loss of light gettting through.

So, light (of all kinds of wavelengths) is travelling through the air. Some of it hits a surface, like a pond, and bounces back towards you. The light that bounces back will tend to be oriented in a particular direction (lets say, for example, only the left/right vibrating light gets reflected back). Light vibrating along a particular axis is called polarized light. Other light will enter the pond, get reflected around under the water and come back out in a basically random orientation.

If you set the filter so that it allows through the left/right polarized light is allowed through, the surface of the pond appears very reflective. This is because (most) of what the filter allows through is what has bounced off the surface. On the other hand, you could turn the filter ninety degrees, and none of the polarized light would get through. This would eliminate surface reflections on the pond; all that would be getting through would be (a little of) the randomly oriented light from under the water. Turning the filter to an intermediate position gives an intermediate result.

The same kind of idea applies to reflective buildings, tree leaves, and clouds — all of which can be enhanced using a polarizing filter. Note for digital photographers: unlike a skylight or tungsten balancing filter or any filter that changes the colour of the image, a polarizing filter is one effect that can’t be duplicated in an image editing program.

Now, back to the circular polarizing filter. With a linear polarizing filter, the light coming out of the filter passing to the lens is all oriented in a particular direction. Unfortunately, the autofocus systems of your SLR uses light in different orientations to detect whether the subject is in focus or not. This means that you can’t autofocus with a linear polarizing filter on you camera. To get around this, another layer is added to the filter (between the camera and the polarizing layer) which gives the light a little twist as it exits. Light leaving the circular polarizer will then have an assortment of orientations, and the autofocus system works. (Note that this doesn’t defeat the purpose of the filter because it has already eliminated light with an incorrect orientation, and the orientation of light hitting the film/sensor doesn’t matter.)

Wow. This is longer than I intended. It’s also longer than your article. Oh well, enjoy.

E.