"See, how they work," the first said, "is the right lens is polarized vertically and--"

"Wait, what do you mean by 'polarized'?" said the second.

"A vertically polarized lets no horizontally polarized light in, and a horizontally polarized lens lets no vertically polarized light in," said the third. "If two perpendicularly polarized lenses are put together, no light is let through."

"So as I was saying, the right lens is polarized vertically, and the left horizontally, or vice versa, such that each eye sees a different picture."

The second had an idea. "So that means if I..." He closed his right eye, and smiled. "Yes, I can only see your right eye."

The fourth and final person (you, of course), sensed something wrong. First, you verified the second's observation. Then, you said...

Each eye sees a different picture only if two pictures, one horizontally polarized and one vertically polarized, are being projected. This is the basis of three dimensional movies. each picture being projected is first filmed with an offset equal to the spacing of the normal human eyes.

But of course the lenses don't block out anything illuminated with normal unpolarized light since there is enough light in all planes to see through a single polarized lens. The light level may be reduced a little and this is why you have to open the iris of a camera a stop or two when using a polarized filter to reduce glare in bright sun.

I would however be able to look through either lens and see both eyes of a person wearing a pair of polarized lenses designed for three dimensional viewing. Unless of course I was also wearing the same type of lenses.