Paula Pratt, age 13, of Cleveland, Ohio, for her question:
HOW DO WE SEE?
Vision is the ability of the eye to form an image of an object and send that image to the visual centers of the brain. The process is very much like taking a picture with a camera.
With vision, light rays from the object pass through the cornea, the aqueous humor, the pupil, the lens and the vitreous humor and finally reach the retina. When light hits the light sensitive cells of the retina, it stimulates the nerve cells. The optic nerve carries the message to the visual cortex, or seeing part of the brain.
When light rays hit the curved surface of the cornea, they are partially bent toward each other. This bending of light rays as they pass from one substance to a substance of different density is called refraction.
After leaving the cornea, the light passes through the aqueous humor, then through the pupil. The iris regulates the amount of light that enters the eye by changing the size of the pupil. If dim light enters, the pupil expands to admit more light. If the light is bright, the pupil contracts to shut out unnecessary glare.
Light rays that pass through the pupil bend toward each other still more as they pass through the curved surfaces of the lens. After passing through the vitreous humor, the rays hit the retina.
The point where the light rays should meet is called the focus. If the focus is located on the retina, a clear image is produced. But if the focus falls behind or in front of the retina, the image will be blurred.
When the light rays finally reach the retina, they pass through the nerve cells to reach the rods and cones. The cones lie mainly in the macula, a small area near the center of the retina. The cones do most of our seeing in daylight or fairly bright light. The rods produce vision in dim light.
The rods lie toward the edges of the retina, and the cones are near the center. Nerve fibers attached in front of the rods and cones come together from all parts of the retina to form the optic nerve. This large nerve passes through the retina and other parts of the eyeball and then goes to the brain.
The brain does not see a light until about a 10th of a second after the light is turned on. The image then persists or lasts about a 10th of a second after the light is turned off. This persistence of the visual image explains why a glowing match stick seems to leave a trail behind it when it is thrown away in the dark.
A motion picture projector throws about 24 still pictures on the screen each second. But we see a continuous movement. Each picture on the screen is presented to the eye before the previous image in the brain fades out.