Randall Thomas Hill, age 9, of High Point, North Carolina, for his question:
How does a camera take pictures?
Nowadays almost every family has at least one camera and this is the season for taking pictures of sunny vacation days. A camera works somewhat like the human eye. The best modern cameras perform wonders. But even they are no more than rather clumsy copies of the human eye. Both have parts that work more on less in the same way. But the camera is a man made machine; the eye is a living miracle.
The human eye has an eyelid that opens to let in the light and closes to keep it out. A camera's eyelid is a shutter. When it clicks open, it lets in the light ¬and the light brings inside a picture image of the scenery outside. The colored part of the eye is the iris. Actually it is a delicate muscle around a hole called the pupil. When the light is very strong, the iris makes the size of the pupil shrink. This lets in less light. In the dim evening light, the iris relaxes and the pupil grows wider to catch every glimmer.
The camera also has an iris. It is a circle of overlapping metal plates around a hole in the center. These plates shift when you set the camera for sunny or cloudy scenes. The hole in the center shrinks to cope with vivid sunshine and opens wider for dimly lighted scenes.
The eye has a lens, just behind the pupil. It is a small saucer of glassy material with delicate muscles to pull it out thin or push it together and make it thicker. The changing shape of the eye lens brings near objects and distant objects into focus. The camera too has a lens. It is a magnifying glass, that cannot be made thicker or thinner. But fixtures inside the camera can move it back and forth to focus on objects near and far.
Light comes through the pupil of the eye and places its picture image on a special screen inside the eyeball. This screen is the sensitive retina, with a million tiny nerve cells that relay bits of the picture to the brain. They sort out the scenery into patches of color and keep changing the scenes to create a moving picture.
The camera's retina is a piece of film in a miniature dark room behind the lens.
This man made film is coated with chemicals that change when even a tiny ray of light falls upon them. It may have zillions of silver bromide molecules, embedded in a gelatine substance. When you click open the shutter,. light from outside comes through the hole in the iris and falls on the film. Actually the picture image is a patchwork of light, dark and medium spots. Different patches made different changes on the film and together these pieces make a copy of the scene.
Rays of light change the film in rather an odd way. The brightest rays create the darkest spots. The dimmest rays create the lightest patches. So the picture on the film is a negative, with the light and dark pieces in opposite places. The negative is kept in the dark until it is soaked in special chemicals that stop the light from making any more changes. Then it is placed on printing paper. More chemicals are used to make a copy with the pieces in their proper places. This permanent photograph is the camera snapshot of the scene.