How does photographic film work?

The magic of photography is based on a simple trick of chemistry. Certain chemicals are sensitive to light and we have tamed them to take pictures. We know of certain dyes that are sensitive to 1ight. A pair of blue jeans, for example, may fade after days in the sun. The light sensitive chemicals used to make film are far more dramatic than these everyday dyes. But they, too, change when exposed to the light.
The magic chemical used on a strip of film is usually silver bromide. It is made of molecules which in turn are made from atoms of silver and bromine. If this chemical is kept in the dark, its molecules remain locked in tiny units. But it is highly sensitive to white light, to sunlight or the beam of an ordinary lamp.
When a beam of white light strikes the light sensitive chemical silver bromide, some of its energy is used to unlock the tiny molecules.
They become separate atoms of silver and bromine. This is the  trick used to take a picture on film. The rest of the operation is the taming of this light sensitive chemical. It must be put where we want it, protected until we need it and then fixed to give us a permanent record of the scenery.
The roll of film is a strip of clear plastic, firm but not too firm. One side is coated with gelatin teeming with molecules of silver bromide. The roll is wound tight and sealed from the light. It is made to unwind on a pair of spools inside the camera. When you turn the knob whi.;h controls the spools Just so far, a sheet of film, coated with silver bromide is facing the lens. While the lens is covered, the film waits safely inside the dark camera.
When you press the picture button, the shutter moves back from the lens and, for a fraction of a second, in streams the light. It carries an image of scenery with its light patches and dark patches and this patchwork image falls upon the silver bromide. Where the light is strong, many of the molecules split into atoms of silver and bromine. When the light is weaker, fewer of the molecules are split.
In a flash while the shutter opens and shuts, the film captures a code picture of the scenery outside. The exposed film is unrolled :in a dark room and washed in chemicals. The picture is developed and a loose silver atoms are fixed firmly in the gelatin. The loose atoms of bromine and the whole molecules of silver bromide are washed away, leaving light patches of clear gelatin to show the dark areas of the picture. This is the film negative, light where it should be dark and dark where it should be light. Similar chemicals are used to transfer the negative to print paper, which shows the light and dark areas in their proper places.

The sensitive chemicals on the white print paper react slower than those on the film. The negative is placed flat on the paper and exposed to the light. The dark areas on the negative keep out the light and the paper below remains white. The clear areas on the negative let through the light and the print paper turns dark. The true picture forms on the print. It is then washed in chemicals to stop further change and the photograph i s permanently fixed.