Cinema in EveryDay Science

Cinema: is based on the principle of persistence of vision according to which the impression of an object upon the retina lasts for one-sixteenth of a second. The photographs of Successive positions of the moving object are taken on a transparent photographic film at a rate of 20 per second. The light is then cut off from the screen for 1/80th of a second and the film is drawn across the focal plane and replaced by the next portion. This remains stationary for the next 1 /20th of the second and the process continues. By this intermittent passing of the film across the focal plane of the projection lens, the background is always seen in the position on the screen and continuity is maintained.