A while ago I got a polarised filter for my camera to see what it was like. Obviously it cuts down the light by half, but it also seems to make pictures a bit different, and generally quite nice. However, that is not what I am posting about specifically.
What I got was the "Hoya PL-C" model. Indeed I could not see any linear polarised filters. Wikipedia has a nice bit on circular polarisation.
There are basically two main ways to do things - linear polarisation has a direction, e.g. vertical, horizontal, etc. This is also what is usually used on a simple LCD display such as a digital watch. The light goes through, say horizontally polarised, and is reflected back, and will come through the polarisation again the same way, but the liquid crystal can twist the light so it is vertically polarised and so not come back through the horizontal filter.
If, in the digital watch, you were to have a circular filter, e.g. "right handed", when reflected off the back, it would always clash with the filter coming back, list like a mirror image of screw would not fit in to a nut as the thread is reversed.
Light is usually a mix of different directions, and so a filter normally simply cuts half of it. But some reflective surfaces will reflect a specific polarisation, and by using a filter you can cut out that specific reflection. Clever really.
Some 3D glasses use horizontal for one eye and vertical for the other - you can tell as the image breaks if you tilt your head. Better 3D glasses and projection systems are done using right handed and left handed circular polarisation - this works the same at any angle (though the stereo effect breaks a bit if you turn your head sideways, the images stay with the correct eyes). If you look in a mirror with such glasses on and cover one eye, you can see your eye (the uncovered one) fine if using linear, but black is using circular.
What puzzled me about the filter I got is the ring is designed to turn freely, allowing me to rotate it. This makes no sense on a circularly polarised filter, as right handed is always right handed at whatever angle you turn the filter.
It does, however, make perfect sense if it is a linear filter, as I may want to change to horizontal or vertical... So I pointed it at my watch, and turned it...
Basically, I think, this means I have a linear polarised filter, not a circular polarised filter. How odd.
P.S. thanks for the comment with the wikipedia link https://en.wikipedia.org/wiki/Polarizing_filter_(photography) which explains it is linear and a bit of circular. So why sell as "circular" when it is the linear that matters for the artistic effect!
You do learn something new every day :-)