Cheryl Benton, age 12, of Shawnee, Okla., for her question:
How do we see colors?
Your two bright eyes take moving pictures of the scenery around you. They can bring in the image of a distant mountain, concentrate on the fine detail of a printed page or adapt themselves to see in a dim light. And best of all, when the light is good, they bring all these visual wonders to us in rainbow colors.
Light and electricity are different forms of energy. Sunlight is an invisible blend of rainbow colors, and we cannot see it until something happens to change it. Electricity is the busy activity of tiny electrons rushing to and fro among countless tiny atoms. This form of energy can be sensed by delicate nerves in the human body. When we behold a colorful scene, millions of nerves in the retina of the eye change the energy of light into the energy of electrical impulses.
Each of the colors of light pulses along on a different wavelength. The surface of a red rose absorbs all but the red rays of the rainbow spectrum and bounces the red rays back to reach our eyes. A viola reflects the blue rays and a marigold reflects only the yellow rays for our eyes to see. These and other different strands of white light pass through the lens at the front of the eye and strike the sensitive screen that lines the back of the eyeball.
This screen is the retina. It is covered with countless nerves that act as miniature detectors. The nerve threads are gathered together in one big optic nerve that carries countless scraps of information to the brain. The details are sorted out at optic headquarters in the brain to give us a completed picture of the scenery.
The detector cells are of two kinds. Rod shaped detectors take care of night vision. Cone shaped detectors take care of daytime vision. The cones detect clear shapes and outlines and make it possible for us to read a page of print. They also detect colors. They catch the different wavelengths of light falling upon the retina and translate them into different electrical impulses.
Each cone is linked to a nerve fiber that sends impulses to the brain. Millions of cones go to work when we see a colorful scene, and the brain receives all their sensitive impulses. The different electrical impulses are translated into colors by the brain, and in a split second we see an image of the scenery in full color.
The cones must have light in order to do their work, so they are not much help to us after dark. In a dim light the rod cells of the retina go into action. They manufacture a substance called visual purple that helps us to see the dim outlines of the furniture in a darkened room. But cones cannot detect color and cannot help us see in pitch-blackness where there is no light at all.