Basically, what happens in a 3D film is that the screen is showing two images, and when you wear 3D glasses they make the images enter different eyes – ie one goes into each eye. There are two main ways that this is done:
The older system is known as the red/blue or red/green system. This was commonly used in movies in the past, but is now more often found in 3D television effects. With this system, the two images that are shown on the screen are in different colours – one red and the other either blue or green. Your 3D glasses have filters on them that cause only one colour image to enter each eye, which is what produces the 3D effect (if you look at a pair of glasses of this kind, you will see that they have one red lens and one in either blue or green). However, this older method doesn’t produce a very good quality image.
In more modern 3D cinemas, the more often used method is called polarisation. At Disney world and similar locations showing 3D effects, polarised lenses (which can eliminate reflected light off certain surfaces) are used in the 3D glasses to allow colour viewing. The images which appear on the screen come from two different projectors – each of these is polarised in a different way, so that the light/reflection reduction works differently and you are effectively seeing a different image in each eye. You can actually buy 3D projectors for home use.
In another method, the two images are displayed alternately, with one quickly following the other. For this you need special glasses, not 3D but LCD (liquid crystal display) with lenses that shut off the view from each of the alternate screens in turn rather than feeding a different image into each eye. You can use these at home, but the method and the glasses are expensive, plus you need other special equipment, so this method is less popular than the other two.
Although it’s only in recent years that 3D technology has become very well known, it has been around for a long time. In fact the first full length film using 3D was made in 1922 (it was called The Power of Love). It didn’t catch on very well, and it was probably the 1950s before 3D got any great popularity with the rise of special effects, especially in the horror movies and sci-fi that were so big in that era.
3D is really based on the way our eyes work. When we look at something we’re not usually conscious of looking at it with two eyes, but if you cover one eye with your hand you soon notice that the vision from each eye is slightly different, and our ‘normal’ vision comes from our brain absorbing the information it gets from each eye and processing it to give a picture of the environment we are in.
This is the way a stereoscope (which is what 3D technology is derived from) works. With a stereoscope, each eye looks through a different lens which makes the thing you are looking at seem larger, and also seems to alter its horizon, which means that the two images rather than seeming separate appear to merge into one. If you use a stereoscope, such as a ViewMaster, you can see how this works. This technology itself is based on the way our human (binocular) vision works since, as we saw earlier, each of our eyes also sees a separate image and the two are then processed by the brain. If you think about the way images ‘jump’ when you cover/uncover first one eye and then the other, you will get an idea of how 3d works.
You can get a good overview of this topic here.