Friday, May 27, 2011

[Science Form 2] Eye On Defects

A person with normal vision sees distant and close objects clearly. To maintain a clear image of the object on the retina, the thickness of one's eye lenses changes, in a process called accommodation.

However, for some people, clear vision is not achieved because of defects. Common vision defects are:
  1. Short-sightedness (myopia)
  2. Long-sightedness (hypermetropia or hyperopia)
  3. Astigmatism
  4. Colour-blindness
  5. Lack of accommodation (presbyopia)


1. Short-sightedness (Myopia)

People with myopia are not able to clearly see objects that are far away. This is because light rays focus in front of the retina, a result from the fact that eye is not able to focus light rays further back, despite maximum flattening of its lenses. Short-sightedness mainly affects a person's ability to look at distant objects where the light rays come straight (or parallel) into the eyes.

Short-sighted eye
Corrected short-sightedness

Short-sightedness is corrected by using glasses with concave lenses, which diverge light rays before they enter the eye.

2. Long-sightedness (Hypermetropia or Hyperopia)

People who are long-sighted can clearly see objects that are far away, but they have trouble seeing objects near them. If you are long-sighted, near objects may be so blurry that you cannot do tasks such as reading or sewing. Long-sightedness occurs when the eye does not bend light correctly. This causes the image of an object to focus behind the retina, instead of directly on it, thus resulting in a blurred image.

Long-sighted eye

Corrected long-sightedness
Long-sightedness is corrected by using glasses with convex lenses, which converge light rays.

3. Astigmatism

Astigmatism is caused by an irregular surface of the cornea, or a lense that is not evenly curved. As a result, the vertical and horizontal rays from the image cannot be focused at the same time. Both images may fall short of the retina, or one before the retina while the other behind it. Hence, terms such as compound hyperopic, compound myopic or mixed astigmatism. A person suffering from astigmatism may see certain parts of an object more clearly than others.

Astigmatic eye

Corrected astigmatism
Astigmatism can be corrected by either wearing glasses with cylindrical lenses, or surgery.

4. Colour-blindness

To understand what causes colour-blindness, you need to know about the cone cells or colour receptor cells in your eyes. These cone cells are on your retina, an area the size of a postage stamp that is at the back of your eye.

You have red, blue and green cones, which are sensitive to those colours and combinations of them. You need all three types to see colours accurately. When your cones do not work properly, or you do not have the right combination, your brain does not get the right message about which colours you are seeing. To someone who is colour-blind, a green leaf might look tan or grey. This defect cannot be corrected because it is hereditary.

5. Lack of Accommodation (Presbyopia)

The term presbyopia means "old eye" and and is a vision condition involving the loss of the eye's ability to focus on both near and far objects. It is a condition that occurs as part of normal aging, because when a person get older, one's eye lense becomes harder and less elastic. The muscles in the ciliary body also loss their ability to contract and relax. Presbyopia occurs gradually over a number of years and is usually noticeable by age 40-45.

Presbyopia is corrected by wearing glasses with bifocal lenses to see near and far objects clearly.



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