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Wednesday
Nov102010

The Physics Of Bad 3D Movies

Imagine a beam from a high powered laser coming straight towards your face. As the beam hits your face, you will quickly discover that it - and more generally light - is electric and magnetic in origin. You will realize this because the electrons in the atoms making up your soon to be fried cells will start oscillating violently as the beam passes through them; and electrons are electrically charged and experience electric and magnetic forces. But from the oscillation of the electrons you will also realize that there is more structure to light than you may have thought: as the light passes through an electron in a certain direction, the electron will jump around and move in a plane transverse to the incoming beam; that is if the beam is coming in horizontally, its effect on the electron can be up-down and/or left-right. To be complete, the electron will also be pushed in and out along the beam for somewhat different reasons, but that motion is irrelevant to my argument. This tells you that light carries with it a two dimensional plane's worth of information: you can arrange for a laser to move the electrons only up and down, or only left and right; or a combination of both. We say light has two polarization states, or two degrees of freedom. Your eyes are not setup to see this additional information in a light beam; hence, this may come as a surprise.

However, we now have technology to mold the polarization of a light beam as we desire. This is done by letting the beam pass through certain special materials - called cryptically polarizers: the emerging beam can be made, for example, to oscillate electrons up and down only, or left and right only. If you polarize a beam left and right, then make it to pass through a polarizer oriented up-down, you can kill the beam. This has many uses, the most recent and sexy one being the emergence of 3d movies and 3d televisions.

You perceive distance and perspective in life because you have two eyes. Each eye sees a slightly different perspective of the world, and your brain puts things together for you for free and hence you perceive distance.  When you want to replicate that in a bad movie, you use a camera with two eyes - or just two cameras mimicking two eyes. You then project the video from each "eye" onto the same screen in such a way that light from one video passes through a polarizer that orients the polarization of the light say up-down; the other, left-right. Actually, one does a slightly more practical setup using so-called circular polarization, but that detail is irrelevant to the physics story. When you view the bad movie on the screen, you then put special eyeglasses: each eye has a polarizer oriented differently and picks up only the image from the corresponding video feed. So, your brain receives two different images, slightly off from each other in perspective - just so that it appears you are immersed in the movie's scene. It's a very simple idea that capitalizes on the fact that there are two degrees of freedom in light and one can package two images in the same beam; and separate things out once again with polarizers in front of your eyes. Unfortunately, all this physics will not improve the quality of some of these recent 3d movies.

The first video demonstrates the basic physics of light polarization. The second video talks about 3D TV technology.

 

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