Janice Stamper, age 13, of Mexico, Missouri, for her question:
How do we see colors?
When the light is bright enough, our eyes see the scenery as a moving picture in full color. They can focus themselves to show the small letters on a printed page. Outdoors they can extend our vision to shorn us distant mountains or the moon and the stars in the sky. They can do any of these things in a split second and they perform their work automatically, with¬out waiting for orders from our conscious minds.
Light and electricity are two forms of energy. The secret of vision lies in translating the energy of light into the energy of electricity.
The energy of light pulses along in an assortment of longer and shorter wavelengths. When its wavelengths are separated, they become visible as the spectrum colors of the rainbow. The wavelengths are separated when light falls on different objects. Some surfaces bounce back short wave¬lengths that show the blue rays, other objects reflect the longer red rays.
The scenery around us reflects the energy of light in a multitude of different colors. These different wavelengths enter the round black pupil, which is the window of the human eye. They fall on the retina, which is a sensitive screen covering the back and side walls in the eyeball. This is where the miraculous job of translating lightwaves into electrical impulses is done.
The retina screen is crowded with sensitive nerves with fibers leading to larger and still larger nerve cables that run through the skull to the brain. Behind each of the nerve fibers on the retina is a sensitive light detecting cell. The cell and its nerve work together to translate a wave¬length of light into an electric impulse and transmit it to the brain.
The human eye has two types of these light sensitive calls. The cones are rather squat cells and millions of them are crowded toward the center of the retina. The rods are taller cells and millions of them are crowded around the sides and edges of the retina.
Only the cone cells are able to detect the colors of the different wavelengths of light. They are stimulated when millions of pinpoints of light come through the pupil and fall upon the central part of the retina. Then millions of cones respond in a split second. Each translates a color wavelength into an electrical impulse and its attached nerve fiber relays the coded signal to the brain.
In a split second, the brain sorts multitudes of these signals from tine cones and assembles them into a color picture of the scenery.
The color detecting cones work continuously when the light is bright or fairly bright. They shut down when the light grows dim. Then the rod cells take over the job of relaying signals to the brain. They cannot work in total darkness and they do not perceive colors. But they manu¬facture a biochemical called visual purple. This strengthens shadowy out¬lines and helps us to side step the furniture in a dimly lighted room.