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The Science of Seeing
Human vision is a wonderful and complex process that is not yet completely understood. It involves the eye, nerves and brain all working together to provide the visual information we see.
The outer shell of the eye, called the sclera, is the white rigid spherical shell that gives the eye its structure. The sclera itself is opaque to allow light into the eye. It merges in the front with the transparent cornea, which is the window of the eye. The cornea has an index of refraction of about 1.37. Immediately behind the cornea is the aqueous humor, a clear watery liquid which supplies the cornea with the nutrients it needs since blood vessels in the cornea would affect the optical clarity.
The pupil is the opening in the center of the iris that controls the amount of light entering the eye. The iris merges with colored connective tissue called the choroid which lines the inside of the sclera. In humans, the pupil is circular whereas horses and goats have a horizontal slit. Snakes, alligators and cats have a vertical slit.
Tiny muscles on the iris automatically adjust the size of the pupil within tenths of a second depending on the light level. It is interesting to note that the pupils of both eyes will open and close in unison, even if only one is stimulated with light. This is due to the consensual pupillary reflex. In addition, our attitude about what we are seeing also influences the size of the pupil. This effect, common when viewing pictures of attractive members of the opposite sex, can affect the pupil size by up to 30 percent.
Research illustrates that we are even subconsciously aware of pupil size. Men found a picture of a woman more attractive when the photograph was retouched to make her pupils larger. None of the men studied consciously noticed the difference. Conversely, a sinister, cold, hateful look can be achieved with smaller pinpoint pupils.
Read about the effect of blue light on pupil contraction.
The lens, which is immediately behind the iris, provides fine focusing to adjust for the object distance. This process is called accommodation and is accomplished with a ring of muscles around the lens. When the muscles are relaxed for viewing distant objects, the lens is relatively flat. When the muscles constrict to view objects close up, the lens changes shape, becoming more curved.
The near-point is the closest point where the eye can still focus. This distance increases with age as the lens gradually looses elasticity. This distance usually surpasses the our arm length between the ages of 50 and 60, which then calls for corrective lenses. Cataracts, or a loss of transparency of the lens, also affect many people as they get older.
The inner chamber of the eye is filled with a clear jellylike substance known as the vitreous humor. This structureless substance has an index of refraction close to that of water. Sometimes when you look carefully, you can see bits of cellular debris in the vitreous humor called floaters that give a faint shadow to the image you see.
The retina, or light sensitive part of the eye, covers the back of the eyeball and is the final destination of the light. The lens and cornea actually invert or turn the image displayed on the retina upside down in the process of providing a clear image that is in focus.
How do we see upside down?
Since we have been seeing things upside down since birth, this really isn't any problem at all. In fact, the American psychologist G. M. Stratton experimented with a pair of glasses that inverted the image to make it right side up. He found that he had to "relearn" how to see, a process that took days. In a later unrelated experiment, the participant actually reached the point where they could ride a bicycle while wearing the glasses. The challenge, however, is that the participant's world is again turned upside down when they cease wearing the glasses. Fortunately, it takes less time to return to the normal upside down world.
This is the area of vision where we see clearly. We move our head and eyes so that the image of the object we want to look at falls on the fovea. But because the eye moves around freely, it seems as though we can see everything in our field of view with equal clarity.
The fovea is surprisingly small. It contains only cones and virtually no rods, which partially explains why peripheral vision is better at night. This also explains why it is impossible to see with the same clarity at night no matter how close you bring the object.
The fovea has no blood vessels or nerve cells. It is about 1/5th of a millimeter in diameter and appears as a pit in the retina.
The Difficulty of Seeing Blue
The very center of the fovea lacks S cones, the cones that are sensitive to blue, and there are fewer blue-sensitive cones in the fovea in general. The result is small-area tritanopia, or a yellow-blue color blindness, that we all have for very small objects. This effect is easily left undetected since your eyes are almost always scanning from spot to spot, and your mind has a "blind spot" of sort with regards to the missing blue.
Some researchers have proposed that the reason for the lack of S cones in the fovea is due to a yellowish substance, xanthophyll, that covers the fovea and absorbs blue light, but we don't agree. We subscribe to the belief that since the S cones are slower reacting than the other cones, having blue cones there would slow down the effective eye movement.
Very Few Rods Are In the Fovea
It also turns out that there are relatively few rods in the fovea, making it sometimes difficult to see what we're looking at in poor lighting conditions, since the rods are more sensitive and used for night vision, whereas the cones are color sensitive and used for color vision.
No Blood Vessels
The concentration of cones is so dense in the fovea, there actually are not any blood vessels in the area.
Other Interesting Bits
The fovea is not at the exact "optical" center of the eye, but rather 4 to 8 degrees out.
The Macula, an oval spot near the center of the retina, is responsible for our ability to see details. It contains the fovea and is covered with a yellowish substance, xanthophyll, that absorbs some of the blue light. This absorption protects the retina from being exposed to too much short wavelength light. You would think that, since the macula is covered with a yellow substance, we would see everything in the world with a yellow tint. But we don't, due to the process of chromatic adaptation where our eyesight adjusts colors to perceive them correctly. The density of the yellow filter varies between people and accounts for some of the differences in how individuals perceive color.
The macula and fovea are critical for most every detailed vision task we undertake, whether driving, or reading.
This debilitating disease results in the missing or blurred vision in the central area and is the most common cause of central vision loss in the United States today for people over fifty. Unlike a loss of the peripheral vision, which may go unnoticed, any loss of vision in the macula is immediately obvious.
Myopia and Hyperopia
Myopia or nearsightedness and hyperopia or farsightedness result from a failure of the eye to focus the image correctly on the retina at the back of the eyeball. In myopia, the focal point of the eye is too short, making it difficult to clearly see distance objects, but objects can be seen clearly up close. This condition can be aided with a diverging lens which lengthens the focal distance.
In hyperopia or hypermetropia as it is sometimes called, the focal length is abnormally long, making it difficult or impossible to focus on objects up close. A converging lens is needed to help bring the image into focus.
Red Eye Reflections
Have you ever noticed the reflection of a cat's eyes in your headlights or seen the effects of red eye on a photograph? This occurs because the rods and cones of the retina are actually pointed away from the pupil and toward the reflective surface at the back of the eye. Scientists don't seem to have any explanation for this. Normally the eye appears dark, because you block the light when you look into someone's eye. That's why ophthalmologists used an ophthalmoscope to look through a partially silvered mirror that simultaneously projects light directly into the eye.
Read more about the human eye.
Facts and Fiction
Color and Eyesight
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The information published here is for entertainment purposes only and is in no way intended to dispense medical opinion or advice or to be a substitute for professional medical care, be it advice, diagnosis or treatment, by a medical practitioner. If you feel ill or if you have a medical issue, you should consult a health care professional.