Draw a picture of an empty cage one side of a piece of white cardboard. Draw a picture of a bird on the other side. Tie a thread at the top of the cardboard and suspend it from a hook. Give many twists to the thread by turning the cardboard and then release it. The cardboard rotates rapidly as the thread unwinds and it appears as if there is a single picture of the bird inside the cage. The illusion is due to persistence of vision. The positions of the picture change rapidly and the next picture appears before the impression of the first picture has disappeared from the retina. Both the pictures, thus, are seen simultaneously giving the impression of a bird inside a cage.

Persistence of vision forms the basis of motion pictures and television. If more than 10 pictures, showing successive positions of an object in motion, are flashed in front of the eye every second, new images appear on the retina before previous ones are cleared. The images thus overlap giving the impression of a continuous moving image. Motion pictures and television work on this principle.In a motion picture, photographs of moving objects are taken at the intervals of less than 1/10th of a second. Each picture, thus, is slightly different from the previous one. While viewing, they are projected at the same rate. Between two successive images projected, there is a brief period of darkness when there is no image on the screen. But before the impression of an image has disappeared from the retina, the next image appears. Successive images thus overlap due to persistence of vision and we get the impression of the image of a moving object.