How can I find out if the polarizer is linear or circular? Is there anything else to know about it? All I know about a polarizer is that it's useful when taking pictures of water or windows.
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Rather than using a second polarizer (that you may not have), try out Tip#7 from this forum post:
TIP #7: To distinguish a Circular Polarizer from a Linear Polarizer, turn the filter backwards and look through it into a mirror. If the filter image in the mirror is black, you have a circular polarizer. If the image is clear, you have a linear polarizer.
A circular polarizer is a linear polarizer with an extra piece of material called a quarter waveplate attached. After filtering light according to the direction of polarization the waveplate mixes up the orientations again so the light coming into the lens is unpolarized. This is done to prevent problems with the autofocus sensors.
You can test the filter to find out if it is circular or linear by using a second polarizer of either variety. The light coming through a linear polarizer is polarized and so can be filtered out by a second polarizer when at the correct angle (when turning the second polarizer the view should become very dark at a certain angle).
The light coming through a circular polarizer is unpolarized by the waveplate so you can't filter it out in this way.
If you find your filter is a circular one then you can use it as usual. It will reduce the intensity of reflections from glass or water, and reduce the appearance of atmospheric haze for better looking skies. If it turns out to be linear then you can use it front of a second circular polarizer to make a variable ND filter. You ought to be able to use it on it's own for the same purposes as a circular polarizer, just be aware that it might prevent the autofocus from working in certain circumstances.