Kevin Watson, age 9, of Visalia, Calif., for his question:
HOW DO OUR EYES FOCUS?
Our eyes work like a camera and must refract or bend incoming light rays so that they produce a tiny image from a large one. The adjusting of the rays so that they bend in one way or another is known as focusing.
The cornea and vitreous body of the eyes are fixed in their positions. Each bends or refracts the light rays in an unchanging fashion, unless altered by disease or aging.
The eye lens, however, changes from the roundness of a ball to a more flattened shape, refracting the light to enable us to focus clearly on objects near or far. This change is called accommodation to distance.
Accommodation to brightness is accomplished by the pupil, the part of the eye which can open wide or close narrowly to admit more or fewer light rays.
Another important part of accommodation is the adjustment of the two eyes to bring the two visual images together for a clear binocular or two eyed vision. The use of two eyes allows us to almost look around parts of the objects as we are viewing, giving us clues to the shape of the objects and how far away they are from us. A person with only one eye has trouble judging distance.
The adjusted incoming rays of light are focused on the retina in the back of the eye. Here, cells called rods and cones are stimulated by the light rays to send impulses of the image to the brain via the optic nerve.
The image is received upside down on the retina, but adjustments in the brain enable us to visualize it as right side up.
The cones can detect the wavelengths typical of various colors. Thus color blindness may result if a person is born with defects of these cells.
The rods are important for seeing in dim light. When they are slow to adapt to darkness, the condition is known as night blindness.
Improper refraction with an inability to properly focus is one of the reasons why many people must wear eyeglasses. Alterations in the ability of the eyes to refract and focus light properly take place throughout our lives and are not the result of diseases or misuse of our eyes as much as of inherited tendencies.
The normal eye, as far as refraction is concerned, is called emmetropic. If the eye is abnormal in refraction in any way, it is called ametropic.
Myopia, or nearsightedness, is a type of improper refraction in which the rays of light converge or come together at a point in front of the retina.
In hyperopia, or farsightedness, the rays do not converge at the retina, but would converge behind the retina if not blocked by it.
Astigmatism is a condition in which the cornea is curved more in one direction than in the other, causing improper refraction.