How are movies made ?

We are able to see objects because the eye lens produces images of the objects on the retina. However, the impression of an image on the retina does not disappear as soon as an object is removed from the view of the eye. It lasts for about 1/10th of a second after the object is removed. In other words, we continue to see an object for about 1/10th of a second even after the object has been removed from our view. This effect is called persistence of vision.

Move a burning stick of incense rapidly in a circle. Although there is only one red-hot spot at the tip of the stick, you see a circle of red light. This illusion is due to persistence of vision. At any instant, the red-hot tip of the stick is at a definite position and gives rise to one point-like image on the retina. Corresponding to the different positions of the moving red-hot tip, a succession of images is produced. However each of the images lasts for about 1/10th of a second. Hence, at any time, many images of the spot are present on the retina. All these images together give an impression of a circle of red light.

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.

In a television set, there is a cathode ray tube. An electron beam is made to fall on the screen according to the signal received. The beam falls on the upper left corner of the screen and moves horizontally from left to right. When it reaches the right end it jumps back to the left end, slightly below the initial position, and once again starts moving to the right. This process continues until the beam covers the entire surface of the screen. When it reaches the lower right corner, the beam jumps back to the upper left corner and the whole process repeats itself. This scanning continues at a very rapid rate. The inner surface of the screen is coated with a thin layer of fluorescent material. When the electron beam falls on a certain point on the screen, the point glows with brightness that is proportional to the intensity of the beam. The intensity of the electron beam, in its turn, is proportional to the brightness of the point of the picture being telecast. Consequently different points of the screen glow with different intensity at slightly different times. We, however, see a continuous image due to persistence of vision.