Donald Rollman, age 10, of East Petersburg, Penna., for his question:

How does film make a photograph?

The film in a camera is a strip of transparent plastic or cellulose coated with gelatin. In the gelatin there is a chemical which is sensitive to light. When a ray of light falls upon it, a chemical change occurs and a new substance is formed, As a rule, the chemical used is silver bromide. Millions of these tiny molecules crowd together on every square inch of film. When light strikes them, they are chemically chenged.

These molecules of silver bromide are made of even smaller atoms, same of them atoms of silver and some of bromine. A ray of light makes the molecules break apart into separate atoms of silver arid bromine„ This happens when we click the shutter to take a picture. Light from the outside then comes through the lens into the dark inside of the camera and falls upon the screen of film along the back wall.

This light, however, produces a picture of the scenery. Light objects reflects more light than dark objects. The entire picture is made up from patches of light and shade. Where a patch of very strong light strikes the sensitive film, a great many molecules of silver bromide are broken apart. When the light is week, only a few are broken apart.

Now we send the film to be developed. It is put into a bath of liquid chemicals which fix the free silver atoms firmly in the gelatin, The file: now has another chemical bath which washes away the bromine and all the silver bromide atoms which have not been split apart. It is given a final clean bath and left to dry.

We now have the finished negative of the picture. It is a patchwork of light and dark areas and everything is wrong way around. Where the silver atoms are stuck, we have a dark patch.

Where the silver bromide molecules were washed away, we have a light patch. Remember, it was the strong rays of light reflected from light objects which caused the silver bromide to split.  So, on the negative, we have     light or transparent areas where there should be dark areas and dark areas where there should be light.

All this is set right when a print is made from the negative. The paper for the print is coated with chemicals such as silver chloride which behave very much like those on the original film, They are sensitive to light. The negative is placed on top of this special paper and a strong light is allowed to shine through. A lot of light shines through the transparent patches of the negative. Only a little light or maybe none comes through the dark patches.

The print is then given several chemical baths. The areas where the light was weaks those dark patches on the negative, are now washed clean. The areas where the light was strong, the light or transparent areas on the negative, show up in various dark tones: The print now shows a picture of the scenery as it appears to us.