The sky is blue, the grass is green, the sun a burning yellow, but why? We know that white light is really a composite of the colors orange, yellow, green, blue, indigo and violet. The retina, which receives the light, contains three types of color sensors called red, green and blue cones. The red cones respond to the longer wave-lengths of light, the green cones to the middle range, and the blue cones to the short wave-lengths.
The brain has to juggle all of this different wave-length information and funnel it through the retina’s 5 million cones before we get a precisely painted picture of what our eyes have seen. This constantly changing stream of images from the world to our mind gives patterns of color at speed of light.
This amazing visual system works reliably in all but a tiny percent of the American population. When it does not, the result is color blindness.
The National Cancer Institute states that exposure to strong sunlight, particularly in early childhood, may increase the risk of intraocular malignant melanoma (eye cancer). This disease affects about 2,000 people each year and can be fatal if left untreated.
Being born in the south, having blue eyes, indulging in sun bathing and not wearing sunglasses all appear to increase the risk. Being born in the south increases the risk three-fold. Blue-eyed people had the highest risk. Brown-eyed people were 60% less likely to develop the disease.
Some practical advice: Always shield your youngster’s eyes from the sun with hat or sunglasses. By filtering out the ultra-violet light rays, they also help prevent night blindness. As we get older, the crystalline lens of the eye helps in filtering out these harmful rays. Heed the advice of these experts and avoid undue exposure to sunlight unless you have adequate eye protection.
Many people have more difficulty seeing clearly at night than they should. This condition is called Nyctalopia in which the retina does not regenerate a pigment called visual purple. This purplish-red pigment prevents night blindness but can become bleached out when the eyes are exposed to strong sunlight or if the retina is deficient in vitamin A.
This condition should not be confused with the normal reduction of vision we all experience at night. The reason for this is that as the light gets less, the pupil (that little hole in the center of the eye) opens wider to allow more light to enter. As photographers know, the smaller the aperture, the sharper the picture. So when the pupil gets larger, the “picture” is not quite as sharp.
Not sure? Your optometrist can measure your dark adaptation and determine if you have night blindness. He may prescribe sunglasses to block out infrared rays of the sun or may suggest an increase of vitamin A in your diet.
The normal eye is kept moist by a thin film of tears on its surface. Most of these tears are produced by glands under the upper lid and are spread across the cornea with each blink. If tear production is reduced for any of a variety of reasons, you will suffer symptoms of hot, burning, dry eyes. In very severe cases, the cornea may dry, become cloudy and affect your sight.
This is one of the conditions which your optometrist looks for during an eye examination. If indicated, he can perform a simple test to determine the amount of tear fluid you have. If the amount is moderately low, you can gain comfort by using drops of artificial tears. For a critical shortage, a soft contact lens may be indicated to keep the eye moist.
A dry eye can also result if the tear production is normal, but the blink reflex is missing. When this occurs, tears cannot be spread across the eye.
An optometrist does much more than prescribe vision correction for eyes; he or she checks to see that the eyes themselves are functioning properly.