I went to a museum where they had a 3D show and they handed out some cardboard glasses so we could see the image in 3D. The lenses in the glasses were a sort of polarizing plastic.
We had two sets of glasses, and I was showing my daughter how they cancelled out the light. I put the left lens of one over the right lens of the other and it blocked all light. I put the left lens of one at a 90 degree angle over the left lens of the other and their intersection was black. All that I understand why it’s happening. But then I did something and I can’t explain the result.
I took the left lens of one and put it face-to-face with the left lens of the other and it blocked all light. However, if I put the lens front to back with the other, it is clear. Why does putting the same orientation lens face-to-face block all the light?
Let’s see if I can draw this in ASCII to make this clearer. Say that this is the 3D-glasses looking down from above:
Back of left lens --> } <-- Front of left lens
Back of right lens --> ] <-- Front of right lens
Then I get the following results by stacking the glasses:
}] == Black as expected
]}
}} == Clear as expected
]]
}{ == Black??? Not expected
][
I thought the polarizing filters worked by filtering out light which was not at a certain orientation. So why would the same filters stacked front-to-back let light through, but stacked face-to-face (or back-to-back) they block they light?
Sounds like they are circular polarizers. When you flip them over, you reverse the direction of polarity by 180 degrees, even though the orientation is the same.
I think they are linear polarizers, but not a horizontal/vertical pair as you might expect. Instead they are both 45 degrees from horizontal, in opposite directions. So if you flip a lens, its polarization axis is rotated.
I don’t think it has anything to do with circular polarization. If the 3D system uses circular polarization, then each “lens” on the 3D glasses consists of a waveplate (which converts circular polarized light into a linear polarized light) in front of a linear polarizer. So the light coming out the back side is linearly polarized, and ]} and }] combinations would be clear. (Actually it’d be dark, but not black.)
We just had discussions about this. Look through threads about three months back.
The older 3-D glasses used linear polarizers mounted at 45 degrees to the vertical, but perpendicular to each other. The newer glasses use, I believe, a combination linear polarizer and circular polarizer (one after the other) or linear polarizer and half-wave plate. The result is that, of all the ways to place two such “lenses” next to each other, only one will result in linear polarizer being next to linear polarizer, and when you rotate one against the other, you can vary the darkness you get. In other combinations, you get no change.
You wouldn’t get this effect if they used circular polarizers, as QED claims. The last time I tried this, at Monster House, the “lenses” would darken against each other when you put them the correct way.