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As we know, our body can have different types of diseases which can be cured and improved to some extent. Like this, the human eye, which is the most complex organ of our body, also has certain types of diseases. The disease is related to the vision of the eye or the power of accommodation. In this condition, the viewer cannot see the objects clearly at some distance. Their vision becomes blurred and they also feel some strain in their eye. This is due to the refractive defects of the eye. There are mainly three types of defects of vision that are Hypermetropia or far sightedness, Myopia or nearsightedness, presbyopia. These defects can be corrected by using optical lenses.
The vision problem can be solved by using optical lenses suitable to our refractive index error. These lenses help to focus the light ray on the retina by correcting the focal length.
EYE DEFECT – The human eye loses its power of accommodation. Thus, this inappropriate condition of the human eye causes eye defects. It can also be defined as the disability of ciliary muscles which helps in changing the shape of lens by changing the focal length. These changes may cause problems in the formation of images on the retina.
Eye defects and their correction
The eye defects are of several types :
1) Myopia or nearsightedness
2) Hypermetropia or far sightedness
3) Presbyopia
Now we discuss each defects in detail,
1) Myopia – This defect is also known as nearsightedness. In this defect, a person can see nearby objects clearly but is unable to see distant objects. This defect has a far point nearer than infinity. So, a person with this defect can see clearly only up to a few meters. In a myopic person, the eye forms an image of a distant object in front of the retina. So, when the image is formed in front of the retina, then the viewer is not able to see distant objects clearly. This is due to the high convergence of the eye lens.
When the eyeball becomes too long, the retina gets distant from the eye lens. This defect can develop in childhood and can grow rapidly with the growth of a child.
CAUSES OF DEFECT
This defect may arise due to,
1) Excessive curvature of the eye lens
2) Elongation of the eyeball
3) The protective outer layer is too curved, hence the light entering into the eye cannot focus properly.
4) Occur when the eye has too much focusing power
5) This can occur by spending more time reading, working on a laptop in heavy and bright light. It can trigger myopia.
6) Myopia is commonly seen in families.
7) Short-sightedness in older people can actually be an early indication of cataract.
8) Also occurs if a person spends less time outdoors.
COMMON SYMPTOMS
There are some common symptoms seen in a myopic person,
Strain, fatigue, squinting, headache.
Correction of myopia defect
A person with this defect can use an optical lens to overcome the problem of myopia. They can be corrected by using spectacles, including a concave lens.
Concave lenses help the myopic person. As a concave lens diverges the parallel light rays which are coming from a certain distance. Thus, the virtual real image is formed on the retina. In simple words, a concave lens decreases the converging power of a human eye lens.
Myopic eyes can also be overcome by refractive surgery or by contact lenses. But this surgery is very costly and gives good results.
When myopia is at its peak level, then this stage is called degenerative nearsightedness or high myopia. Once myopia reaches a high level, then it is impossible to treat and can lose their eyesight.
HYPERMETROPIA
It is also called a far sightedness defect. A person with this disease can see far objects but cannot see nearby objects. The person with this defect has a near point far away from the normal point which is 25 cm. So, such people have to keep their reading material or any object beyond 25 cm for clear vision. This defect occurs because of formation of image behind the retina. If an image forms behind the retina , then the person will not be able to see the object properly.
CAUSES OF THIS DEFECT
This defect occurs because the eye becomes too short and the optical component of the eye gets small. That’s why they are not strong enough to focus the incident light ray on the retina. It occurs at any stage, mostly seen in childhood. This defect is due to the imperfection of the eye. Lens cannot become round enough. Thus, they did not have enough power to focus the nearby object. This defect is usually genetic.
In simple words, this defect is cause because,
1) The focal length of eye lens is too small
2) The eyeball is too short.
CORRECTION OF THIS DEFECT
Farsightedness defects can be corrected by using glasses or contact lenses of proper focal length. These lenses can converge the falling light ray on the cornea. In simple words, these lenses can change the direction of lights for the formation of a proper image on the retina. People with this defect have to wear these lenses only while reading, writing, and working on the computer.
Always choose Aspheric lenses especially for stronger prescription. These lenses are thinner, lighter.
The lens used for the correction of this defect is the convex lens. This lens provides the additional focusing power which is required for the image formation on the retina. The convex lens should be of appropriate power for the convergence of light rays which are coming from the cornea. This lens helps the eye lens to make the final image on the retina.
SYMPTOMS OF THIS DEFECT
Strain, fatigue, headache, finding difficulty in concentrating , burning of eye
PRESBYOPIA
As we know, the power of accommodation is usually decreased by the aging. The near point is reduced and they find difficulty in seeing nearer objects. They cannot see close objects clearly and comfortably. This defect is known as presbyopia.
Causes of this defect
This defect occurs due to weakening of ciliary muscles and also by diminishing the flexibility of the eye lens.
Conclusion
From all of the above discussion, we conclude that the human eye has various defects namely myopia, hypermetropia, presbyopia. Each defect can be cured by using suitable lenses. As myopic person should use a concave lense as it diverges the light while a hyperopic person should use a convex lens as it converges the light ray. Here we can conclude that the image should only form at the surface of the retina for clear vision. If images formed front and behind the retina can lead to the formation of blurred images. Sometimes, a person can suffer from both myopia and hypermetropia. Such people use bifocal lenses which consist of both concave and convex lenses. The upper portion consists of a concave lens and the lower is a convex lens.
Nowadays, it is possible to correct refractive error by surgical interventions.
