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20/20 or 6/6 Vision: Learn What It Means
Our ability to "see" starts when light reflects off an object that we are looking at and enters the eye. As it enters the eye, the light is unfocused. The first step in seeing is to focus the divergent light rays onto the retina, the inside of the eye's back surface. Once the light is focused on the retina, it stimulates cells that send millions of electrochemical impulses along the optic nerve to the brain. The brain interprets the impulses, enabling us to see the object.
Light refraction
Light entering the eye is first bent, or refracted, by the cornea — the clear window on the outer front surface of the eyeball. The cornea actually provides most of the eye's optical power or light-bending ability. After the light passes through the cornea, it is bent again — to a more finely adjusted focus — by the crystalline lens inside the eye. The lens focuses the light on the retina. This is achieved as the ciliary muscles in the eyeball change the shape of the lens, bending or flattening it to focus the rays. This adjustment in the lens, known as accommodation, is necessary for bringing near and far objects into focus. The process of bending light to produce a focused image on the retina is called refraction.
Why is refraction important?
Vision problems resulting from an error in how the eyes refract light are relatively easy to correct. Ideally, the light is "refracted," or redirected, in such a manner that the rays are focused into a precise image on the retina. Most vision problems occur because of an error in how our eyes refract light.
- In nearsightedness (myopia), the light rays form an image in front of the retina.
- In farsightedness (hypermetropia), the rays hit behind the retina.
- In astigmatism, the curvature of the cornea is irregular — causing light rays to focus to more than one place so that a single clear image cannot be formed on the retina, resulting in blurred vision.
- And as we age, we find reading or doing close-up activities more difficult. This condition is called presbyopia, which results from the crystalline lens being less flexible, and therefore less able to bend light.
Since changing the refraction of the eye is relatively easy through the use of lenses, many of the conditions that contribute to unclear vision can be readily corrected.
Sensory interpretation
The sensory part of seeing is much more complex than the refractive part. Even with the light focused on the retina, the process of seeing is not complete. For one thing, the image is inverted, or upside down. Light from the various "pieces" of the object being observed stimulate nerve endings — cells sensitive to light, which are known as photoreceptors — in the retina.
Rods and cones
Two specialized types of cells — rods and cones — present in the retina help us to see.
- Rods enable us to see in dim light and to detect motion. They are primarily responsible for night vision and visual orientation.
- Cones provide detailed vision for such tasks as reading or distinguishing distant objects. They also are necessary for color detection.
These cells are photoreceptors. They convert light to electrochemical impulses that are transmitted via the nerves to the brain.
Millions of impulses travel along the nerve fibers of the optic nerve at the back of the eye, eventually arriving at the visual cortex of the brain, located at the back of the head. The image is re-inverted so that we see the object the right way up. This "sensory" part of seeing is much more complex than the refractive part — and therefore is much more difficult to treat when problems occur.
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| EYE CONDITIONS | | | Get the facts you need to know | | | Read More > | |
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| FOR CAREGIVERS | | | Tips for coping with vision issues and problems | | | Read More > | |
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