The reason human brains are able to perceive a series of individual images as one continuous motion is because of a phenomenon called "persistence of vision." (A more complex theory of perception using the perceptual illusions of the phi phenomenon and beta movement is now more commonly accepted by scientists, but the term "persistence of vision" is still used for motion picture reference). In the phenomenon of persistence of vision, a brain will retain an image it sees for a fraction of a second after the image disappears. When many individual images are shown quickly after one another, an illusion of motion is created, as opposed to just seeing a series of individual pictures.
As in photography, the shutter in a film projector is a mechanical device which opens and closes, determining how much light will get through. In photography, the light allows the film to be exposed, while in film projection, the light allows the visual perception of motion. The lowest limit of frames per second that a brain will "see" as motion is approximately 10 fps, such as in a child's flip book. With a movie, however, a frame rate that low will cause "flicker," and 16 fps is therefore considered the lowest threshold for film projection. With the advent of sound in film, 24 fps became the industry standard (silent films had used a variety of film rates).
In order for persistence of vision to work smoothly, however, it is necessary that the light is also interrupted and re-illuminated 24 times per second, so that each individual frame be shown clearly and without overlapping the other frames. It is the shutter that controls the light in this way, creating what we perceive as continuous motion.
Projection begins with a light source. A Xenon bulb (used for its intense brightness and the fact that it is extremely long-lasting) is mounted in front of a curved mirror to concentrate its light, which then passes through a pair of lenses, which concentrate the brightness even further. This light then shines on the lens assembly, through which the film is passing. Each frame of the film must pause for a fraction of a second before moving on. The heat of the light is so extreme, that if the film rests for any longer than 1/24 of a second, it will immediately melt.
Before the light source passes through the film, it is intercepted by the shutter. The shutter interrupts the light during the time when the frames are switching, 24 times per second. Viewers do not notice this interruption, due to persistence of vision. Most shutters are propeller-like (rotating petal shutters), turning with blades that will block the light in synchronization with the sprockets which move the film. There are also gated cylindrical shutters, which have the same effect. Many shutters today have a flicker rate of two times the film's rate--in effect, showing each frame twice. Double shutters are commonly used which rotate in opposite directions, ensuring that the light is being cut off of each frame from the top and bottom. These shutter types cut down any possible flicker effect for the audience.
The history of the technology of film begins with photography. By the 1820s, scientists had discovered the effect of light on certain chemicals, which led to still photography (although exposures of eight hours were required). Also in the 19th century, people were experimenting with the concept of persistence of vision, and in 1834, a British mathematician created the modern zoetrope: A series of pictures on the inside of a revolving drum, which were viewed through slits in the drum, creating the illusion of movement.
Working with a concept proposed by Thomas Edison, his team at the Edison lab created the Kinetoscope in 1891, which combined photography and the zoetrope, by revolving a strip of perforated film over a light source with a shutter. Though not technically a projector (the film had to be viewed individually by looking into the Kinetoscope), it is considered by film historians to be the first step toward motion picture projectors. In 1895, the Lumiere brothers debuted their Cinematographe, which was the first actual projector, in France. The first projection of a movie (rather, a few film shorts) in the United States took place at Koster and Bial's New York Music Hall in 1896, which is now considered by many to have been the birthplace of American film.