New Schneider 40.5mm Non-Rotating Linear True Polarizing Filter 40.5 mm Filters US $135.00

Schneider 40.5mm Non-Rotating Linear True Polarizing Filter 40.5 mm Filters US $59.99

Hama 49mm screw-in Linear Polarizing filter in rotating mount US $15.56

Tiffen 77mm Std Rotating LINEAR POLARIZER Filter 77 mm US $43.49

30.5mm Linear Polarizer Filter Rotating Glass Filter PL for Digital Camera lens US $6.99

28mm Linear Polarizer Filter Rotating Glass Filter PL for Digital Camera Camcord US $6.99

25mm Linear Polarizer Filter Rotating Glass Filter PL for Digital Camera Camcord US $5.99

49mm Linear Polarizer Filter Rotating Glass Filter PL US $6.99

52mm Linear Polarizer Filter Rotating Glass Filter PL US $5.85

55mm Linear Polarizer Filter Rotating Glass Filter PL US $5.89

62mm Linear Polarizer Filter Rotating Glass Filter PL US $7.18

72mm Linear Polarizer Filter Rotating Glass Filter PL US $8.99

77mm Linear Polarizer Filter Rotating Glass Filter PL US $8.99

86mm Linear Polarizer Filter Rotating Glass Filter PL for Tamron 200-500mm Sigma US $14.99

New 48mm Schneider Linear True-Polarizer Filter Rotating Tiffen Filters 48 mm US $150.00

Tiffen 72mm Std Rotating LINEAR POLARIZER Filter US $43.95

52mm Std Rotating LINEAR POLARIZER Filter US $18.99

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I think that it probably was explained to you incorrectly. Let me take a stab at it:

Light is often described as being waves of energy. While this isn't totally true, it's good enough for what we're doing that we're going to assume it to be the case.

Anyway, light waves take on a sinusoidal shape as they travel through the air. Some, however, are moving up and down(relative to the ground), some are moving side to side, and some are somewhere in between.

The idea behind a polarizer is to get everything moving in the same direction-let's say, for our purposes, that we want everything moving up and down.

The way it does this is by having a whole bunch of slots(in our case vertical) that are about the width of a light wave. That way, waves traveling up and down will get through, but light traveling side to side will be completely blocked. For the ones somewhere in between, only the up and down component will get though. So, the waves coming out the back of a polarizer will only go up and down.

Because of how the filter looks on a microscopic level, we sometimes call this a linear polarizer.

Some cameras use special types of mirrors, often called semi-silvered mirrors or beam splitters, in the optical path. A beam splitter allows some light to pass though, while reflecting some. Due to the way they work, a beam splitter also acts as a polarizer.

Virtually all autofocus cameras depend on beams plitters for the autofocus system to work. Some Canon cameras in the '70s and '80s, such as my beloved F1s, use a beam splitter as part of the metering system, and some Canons over the years have even used a beam splitter in place of a conventional reflex mirror(Pellix, EOS-1nRS).

Anyway, one of the fundamental properties of polarizers is that they will black out when arranged so that their slots are perpendicular to each other. So, if using a conventional polarizer, it's possible that, if turned correctly, the polarizer will cause the beam splitters to black out completely.

To counteract that problem, sometimes a second filter is placed behind the main polarizer. The purpose of this second filter is to scramble the waves back to a random orientation, essentially turning the light back into unpolarized light.

This arrangement, for some strange reason, is called a circular polarizer.

Most polarizers for cameras, whether linear or circular, are mounted within a round rotating holder.

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