The human eye sees the world from a slightly different perspective because of the distance of one eye to the other. In the case of presenting a movie in basic 3D format, the 3D glasses, particularly called the anaglyph glasses that feature one part with red filter and the other part with cyan filter, allow the contrasting color for each eye to utilize the process called binocular vision.
The process causes the slight variations done in each of the movie's 3D-converted image to be properly processed by the viewer's brain. Through the color variations, the brain correlates the sets of two-dimensional images seen by each eye into three-dimensional depth.
Although the anaglyph glasses have been widely used in 3D movies for more than five decades and also in some low-budget 3D offerings even today, the polarized glasses are now more popularly used in mainstream 3D movies shown in theaters.
The polarized 3D glasses' role is to restrict or control the amount of light that passes through each eye because each lens has a different polarization. This results to a slight variation on what each eye sees at every given period of watching a 3D movie. As the brain processes the images seen by both eyes, the final image renders as three-dimensional depth.
With 3D glasses that follow the LCD shutter glass system, its primary function is to alternately black out each eye at a time to trigger the slight variations of the images being seen on screen.
Each part of the glasses causes one particular image to enter one eye, then a slightly different one to enter the other eye. Done in rapid succession, this results to the varying images seen by both eyes to be processed and interpreted by the brain as three-dimensional depth.
Whatever type of 3D glasses are used by viewers, regardless of the processes and functions involved in tricking the brain to process slightly different images seen by each eye into three-dimensional depth, the ultimate purpose of the 3D glasses is to make the viewers get involved in the action they see on screen. Unlike in a 2D movie that clearly separates the viewers from the rectangular box where the footage is played, a 3D movie allows viewers to see the scenes as if they are happening right in front of them.
The objects flying off the screen and going towards the viewer's direction can feel so real that it may cause the viewer to instinctively move away from her place to avoid getting hit. This form of cinematic reality and involvement also extends to scenes of creepy characters seemingly reaching out to grab the viewer or an amazing sea creature simply passing by the viewer, which prompts her to extend her arms to touch it.