There are four common defects of vision (or defects of eye).
- Myopia(Short-sightedness or Near-sightedness)
- Hypermetropia(Long-sightedness or For-sightedness)
- Presbyopia
- Cataract
[1] Myopia(Short-sightedness or Near-sightedness)
Myopia(or short-sightedness) is that defect of vision due to which a person cannot see the distance objects clearly(through he can see the nearby objects clearly)
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| Myopic eye person vision |
The far point of an eye suffering from myopia is less than infinite (∞)
The defect of eye called myopia(or short-sightedness) is caused.
- Due to high converging power of eye-lens(because of its short focal length)
- Due to eyeball being too long.
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| Myopia and its correction by using Concave lens |
Note:-For correction of myopia we use Concave lens which reduce converging power.
Example:-The far point of a myopic person is 80 cm in front of the eye. What is the nature and power of the lens required to correct the defect?
[2] Hypermetropia(Long -sightedness or For-sightedness)
Hypermetropia(or long-sightedness or far-sightedness) is that defects of vision due to which a person cannot see the nearby objects clearly(through he can see the distance objects clearly).
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| Hypermetropic eye person vision |
The near-point of a hypermetropic eye is more than 25 centimetres away.
The defect of eye called hypermetropia(or long-sightedness) is caused.
- Due to low converging power of eye-lens(because of its large focal length).
- Due to eyeball being too short.
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| Hypermetropia and its correction by using convex lens |
Note:-For correction, we use Convex lens. It increases the converging power.
Example:-The near point of a hypermetropic eye is 1 m. What is the nature and power of the lens required to correct this defect?(Assume that the near point of the normal eye is 25 cm)
[3] Presbyopia
In old age due to ciliary muscles becoming weak and the eye-lens becoming inflexible(or rigid) the eye loses, its power of accommodation because of this an old person cannot see the nearly objects clearly. This leads to the defect called presbyopia. The near-point of the old person having presbyopia gradually recedes and becomes much more than 25 centimetres away. Actually, presbyopia is a special kind of hypermetropia. We can call it old age hypermetropia.” presbyopia defect is corrected in the same way as hypermetropia by using spectacles having convex lenses”.
Corrected by:↛ Convex lens
Note:-
- It is also possible that the same person has both the defects of vision(myopia as well as hypermetropia) corrected by (Bifocal lens having both convex and concave lens)
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| Spectacles having bifocal lenses |
- These days, it is possible to correct the refractive defects of the eye(such as myopia and hypermetropia) by using contact lenses or by undergoing surgical procedure. Then there is no need to wear spectacles.
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| contact lens |
[4] Cataract
The medical condition in which the lens of the eye of a person becomes progressively cloudy resulting in blurred vision is called cataract. This is a defect of the eye which usually comes in old age. Cataract develops when the eye-lens of a person becomes cloudy(or even opaque) due to the formation of a membrane over it. Cataract decreases the vision to the formation of a membrane over it. Cataract decreases the vision of the eye gradually. It can even lead to total loss of vision of the eye.
Corrected by→Surgery
Note:-The eye-defect” cataract” cannot be corrected by any type of spectacle lenses.