about 3-d glasses

The magic of movies in 3d is mostly performed by the human eye.  Most people have two eyes roughly five centimeters apart horizontally and almost no vertical distance between them.  Each eye then sees a slightly different looking image.  Notice that if you hold your thumb in front of you and then close one eye and then switch to the other eye, you see two slightly different images of your thumb.  So at any rate, the brain takes the two images the eyes receive and manages to somehow calculate the 3 dimensionality of the image based on this horizontal difference between the two eyes, thus giving the average human decent depth perception.  How that works is something far more complicated than I can fathom.  Furthermore it is simply not understood yet.  So then someone figured out that depth perception is based on this horizontal difference in the images.  So someone thought that if two images were created from slightly different angles, and each eye were only allowed to see one of the images, then the brain would do its normal magical calculations and put the two images together creating a “false” depth perception.  So the rest is done with the glasses.

The two images can be created in red and blue, and then the glasses have two different lenses, one which filters only red and one that filters only blue allowing each eye to only see one image.  That is the old way, it limited the quality of color in the picture since it involves relying on the color of images to separate the images in the glasses.  The newer way is having the two images, assuming they are on a screen or projector, be polarized.  A quick bit on polarization.  Natural light is unpolarized meaning that the “waves” are oscillating in all directions, up and down, left and right, diagonally, and every way in between.  Polarized light is light that has a certain orientation, only up and down or only left and right or only diagonally etc.  So the two separate images are polarized differently.  One image is polarized vertically, and one horizontally.  Then the two lenses in the glasses are polarized also.  One only allows horizontal light to pass and the other only allows vertical light to pass.  Imagine trying to pass a horizontal bar through a vertical picket fence.  This is the basic visual example of light polarized horizontally trying to pass through a vertically polarized lens, it just will not go through.

So whether using polarized lenses and images or red/blue lenses and images, each eye only sees one image, and the images are from slightly different angles so the brain then brings the two images together and you perceive that the object is flying towards you or something nifty like that.