Reader Question: How does 3D work?

Isabel B of Salem, NJ asks "How does 3D work?"

Normally, you have no trouble telling the difference between an actual object and a picture of that object. You know that the object is three-dimensional (it has height, width, and depth), but that the flat picture is two-dimensional (it only has height and width).

But how do your eyes and brain tell them apart? And how do 3D movies or pictures work?

First, let's talk about how your brain know that an object is three-dimensional and do some simple experiments. Pick an object in the room and look at it while moving your head around. Do you see how your view of the object changes? Now, keep your head still and look at the object with both eyes. Then close one eye. Does it look different, even though your head hasn't moved? Now close that eye and open the other. What do you see? This little experiment shows that your two eyes do not see three-dimensional objects in exactly the same way. When you look at something, your brain compares the slightly different images it gets from your two eyes. This lets your brain recognize when an object has a three-dimensional shape, with depth, as well as height and width. If you do the same experiment while looking at a picture, the images don't change in the same way.

3D movies and pictures work because they make your brain see two different images, which tricks it into thinking that the movie or picture has three dimensions! To get the most common 3D effect, you wear glasses with lenses of two different colors, usually red and green. First, let's talk about a 3D picture. If you look a the picture without the 3D glasses, it looks weird - there's more than one copy! If you have a simple 3D picture, try this. Close one eye and look carefully at the picture through the red lens, then look at it through the green lens. Notice that one copy is invisible through the red lens and the other is invisible through the green. So when you put the glasses on in the normal way, each eye sees a different copy. Two different images are sent to your brain which thinks, "I'm seeing two different images, so the object I'm looking at must be three-dimensional." 3D movies work in the same way.

Silly brain - it's so easy to fool!