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Jai Ganesh

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Registered: 2005-06-28

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Defects of Eye

Defects of Eye

Gist

There are mainly three common refractive defects of vision. These are (i) myopia or near-sightedness (ii) Hypermetropia or far – sightedness (iii) Presbyopia. Myopia is also known as near-sightedness. A person with myopia can see nearby objects clearly but cannot see distant objects distinctly.

Summary

Defects of the eye:

Myopia: (nearsightedness) This is a defect of vision in which far objects appear blurred but near objects are seen clearly. The image is focused in front of the retina rather than on it usually because the eyeball is too long or the refractive power of the eye’s lens too strong. Myopia can be corrected by wearing glasses/contacts with concave lenses these help to focus the image on the retina.

Hyperopia: (farsightedness) This is a defect of vision in which there is difficulty with near vision but far objects can be seen easily. The image is focused behind the retina rather than upon it. This occurs when the eyeball is too short or the refractive power of the lens is too weak. Hyperopia can be corrected by wearing glasses/contacts that contain convex lenses.

Astigmatism: This defect is when the light rays do not all come to a single focal point on the retina, instead some focus on the retina and some focus in front of or behind it. This is usually caused by a non-uniform curvature of the cornea. A typical symptom of astigmatism is if you are looking at a pattern of lines placed at various angles and the lines running in one direction appear sharp whilst those in other directions appear blurred. Astigmatism can usually be corrected by using a special spherical cylindrical lens; this is placed in the out-of-focus axis.

Eye related problems:

Cataracts: A cataract is a clouding of the lens, which prevents a clear, sharp image being produced. A cataract forms because the lens is sealed in a capsule and as old cells die they get trapped in the capsule, with time this causes a clouding over of the lens. This clouding results in blurred images.

Age-related macular degeneration (ARMD)

This is a degenerative condition of the macula (the central retina). It is caused by the hardening of the arteries that nourish the retina. This deprives the retinal tissue of the nutrients and oxygen that it needs to function and causes a deterioration in central vision.

Glaucoma:   The eye produces a clear fluid (aqueous humor) that fills the space between the cornea and the iris. This fluid filters out through a complex drainage system. It is the balance between the production and drainage of this fluid that determines the eyes intraocular pressure (IOP). Glaucoma is a disease caused by increased IOP usually resulting from a malfunction in the eye’s drainage system. Increased IOP can cause irreversible damage to the optic nerve and retinal fibers and if left untreated can result in a permanent loss of vision.

Details

Sometimes a person’s eye has troubling focusing a picture on the retina due to age factor. The human eye loses its power of accommodation and in such conditions, a person cannot see objects distinctly and comfortably. The vision is blurry due to the refractive defects of the human eye. These eye defects can be called defects of vision. These defects can be corrected by using suitable lenses. This Physics article will go through different defects of vision and their correction.

Defects of Vision And Their Correction

If a person cannot comfortably and clearly see distant or nearby objects, the person is said to have defects in their eyes. Defects in the eye can occur due to various reasons, including age-related changes and alterations in the focal length. One common eye condition is cataract, where the lens becomes cloudy, leading to partial or complete vision loss if left untreated. Fortunately, cataract surgery can restore vision.

Another issue is the loss of the eye's ability to adjust its focal length, resulting in problems like blurred vision, and difficulty in seeing objects nearby or far away. When there are irregularities in the refractive index, it affects the clarity and comfort of vision. Timely care is crucial to prevent a complete loss of the eye's accommodative power.

Let us understand the concept a bit more in detail. Here, we are going to learn about a few defects of vision in the human eye and also study their correction, like

* Myopia
* Hypermetropia
* Presbyopia
* Cataract
* Astigmatism
* Colour blindness

But first, it is necessary to understand the structure of the eye and power of accommodation of the eye.

The Human Eye

The human eye is one of the most valuable and sensitive organs of the body. It enables us to have the power of vision.

One can compare the human eye to a camera. Its lens forms an image on the light-sensitive screen called a retina. The retina is a delicate membrane containing a large amount of light-sensitive cells.

The light enters the human eye through a thin membrane called the cornea. It forms a transparent bulge in front of the eyeball. The eyeball is almost spherical in shape. It has a diameter of about 2.3cm. Most of the refraction of light rays entering the eye occurs on the outer surface of the cornea. The crystalline lens provides the finer adjustment of the focal length to “focus” at different distances on the retina.

There is a structure called the iris, which is right behind the cornea. Iris is a dark muscular diaphragm which controls the size of the pupil. The pupil regulates and controls the amount of light entering the eye. The eye lens forms a real inverted image of the object on the retina. These cells are activated upon illumination and generate electric signals. The signals are sent to the brain via the optic nerves. The brain interprets these signals and processes the information generated via the eclectic signals to perceive the objects as they are.

Since we are learning about the human eye and defects and vision and their correction, we also need to learn the concept of the power of accommodation in brief for a better understanding.

Power of Accommodation

The ability of the eye to adjust its focal length is called the power of accommodation of the human eye. The eye lens is composed of a fibrous jelly-like material. Its curvature can be modified to some extent by the ciliary muscles and the change in the curvature of the eye lens can change the focal length of the human eye.

* When the muscles relax the lens becomes thin. Hence, the focal length of the eye increases.
* When you are looking at objects closer to the eye, the ciliary muscles contract. This increases the curvature of the eye lens and it becomes thicker and the focal length of the eye decreases. This enables us to see the nearby objects clearly.
* However, the focal length of an eye cannot be decreased below a certain limit.
* The minimum distance at which the object can be seen more distinctly without any strain is called the least distance of distant vision. It is also called the near point of the eye.
* For a young adult with normal vision, the near point is 25 cm.
* The farthest point up to which the human eye can see clearly is called the far point of the eye.
* You may note here that the normal eye can see objects clearly that are between 25 cm and infinity.

Defects of Vision and their Correction

There are a few common defects of vision like

* Myopia
* Hypermetropia Correction
* Presbyopia
* Cataract
* Astigmatism
* Colour blindness

These defects can be corrected by the use of spherical lenses. We will discuss these topics briefly below.

Myopia

Myopia is nearsightedness. A person with myopia can see nearby objects clearly but cannot see distant objects distinctly. A person with this kind of defect has a far point near infinity. Such a person may see clearly up to a distance of a few metres. In this case, the image of a distant object is formed in front of the retina and not on the retina itself.

Cause: This defect may arise due to excessive curvature of the eye lens and elongation of the eyeball.

Correction: This defect can be corrected by using a concave lens of suitable power. A concave lens of suitable power will bring the image back onto the retina and thus the defect can be corrected.

Hypermetropia

Hypermetropia is also known as farsightedness. A person with hypermetropia can see distant objects clearly but cannot see nearby objects clearly. The near point for a person is farther away from the normal near point which is beyond 25 cm. Such a person is required to keep the reading material much beyond 25 cm from the eye for comfortable reading.

Cause: The focal length of the eye lens is too long or the eyeball is too small.

Correction: Convex lenses of appropriate power can correct a hypermetropic eye. Eyeglasses with converging lenses provide additional focusing power that is required for forming the image on the retina.

Presbyopia

The power of accommodation of the human eye decreases with ageing. For most people, the near sight generally fades away. They find it difficult to see nearby objects comfortably and distinctly without eye correction glasses. This defect is called presbyopia.

Cause: It arises due to the gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens. Such a person may suffer from both myopia and hypermetropia.

Correction: Such people often require bifocal lenses to consist of both concave and convex lenses. The upper portion consists of a concave lens, which facilitates distant vision. The lower portion consists of a convex lens, which facilitates near vision.

Cataract

Cataract is a condition in which the lens turns opaque and the vision is cut down to almost total blindness. In cataract, the cloudy lens is positioned behind the iris. As cataract continues to develop further, the clouding becomes even dense. This prevents a sharply defined image from reaching the eye retina, hence blurring the resultant image out.

Generally there are four types of cataracts:

* Nuclear cataracts, which affect the centre of the lens.
* Cortical cataracts, which affect the edges of the lens.
* Posterior subcapsular cataracts, which affect the back of the lens.
* Congenital cataracts, cataracts which you are born with.

Cataracts can generally be corrected by surgery and can reduce the risk by wearing appropriate eyewear and taking the right care of the eyes.

Astigmatism

Astigmatism happens when the light fails to enter the retina with a single focus to produce a clear distinct vision.

In astigmatism, there are multiple focal points in front of the retina and behind. It generally happens due to an irregularly shaped cornea, the front surface of the lens inside the eye has mismatched curves.

Colour Blindness

The retina of the human eye consists of light-sensitive rods and cones. The rods respond to the intensity of light whereas the cones respond to the colours.

Due to certain genetic disorders, a person has some irregularly shaped cone cells that respond only to a certain wavelength of light. Such a person will be unable to respond to few other colours and this itself is known as colour blindness. Colour blindness cannot be cured but one can correct it by using colour-correction lenses/glasses.

Additional Information

The human eye is a wonder of biological engineering because it gives us the ability to see and understand the environment around us. However, the eye is prone to some flaws and anomalies that can obstruct vision, much like any complicated system.

In this post, we explore some of the most prevalent eye problems that impact millions of individuals worldwide as we dig into the intricate workings of the human eye, illuminating its structure and function.

1. Anatomy & Function Of The Human Eye:

The human eye is a sensory organ that gives us the ability to see by catching the light and turning it into electrical impulses that our brain interprets as visual representations. The cornea, iris, lens, retina, and optic nerve are some of its major components. The retina, which includes specialized cells called photoreceptors that sense light and send messages to the brain through the optic nerve, is illuminated by light focused by the cornea and lens onto the retina. Our ability to see is based on this procedure.

2. Myopia (Nearsightedness):

Myopia, sometimes referred to as nearsightedness, is one of the most common eye conditions. Myopia is a condition in which the eye's focusing capacity is very powerful, blurring distant objects while keeping close items distinct. Myopia frequently results from an enlarged eyeball or severe corneal curvature, which causes the light's focus point to miss the retina entirely and land in front of it instead. Myopia can be cured by wearing glasses, using contact lenses, or having LASIK or other refractive procedures.

3. Hyperopia (Farsightedness):

The opposite of myopia is hyperopia, sometimes referred to as farsightedness. Because the eye's concentrating strength is inadequate in those with hyperopia, nearer items look fuzzy while distant ones are quite clear. A shorter eyeball or a flatter cornea are the usual causes of hyperopia, which causes the light's focus point to miss the retina. Hyperopia can also be treated with glasses, contacts, or refractive surgery, much like myopia.

4. Astigmatism:

A common eye ailment known as astigmatism is characterized by an abnormal corneal or lens curvature. Vision at all distances is warped or blurred as a result of this irregularity's unequal light focus. Astigmatism patients may encounter symptoms including headaches, eyestrain, and trouble focusing. The use of glasses, astigmatism-specific contact lenses, or refractive surgery are all examples of corrective methods.

5. Presbyopia:

Presbyopia is an age-related eye disorder that mainly affects people over the age of 40. It happens as a result of the lens's natural hardness and lack of flexibility, which makes it harder for the lens to focus on adjacent objects. As a result, people with presbyopia could have trouble reading or doing close-up work. Reading glasses, bifocals, multifocal contact lenses, and surgical procedures are all available as corrective measures.

In conclusion, The eye is a complex and amazing organ in the human body that gives us the gift of seeing. But different eye conditions can impair visual clarity and quality in people of all ages. Regular eye exams, early identification, and the right remedial methods may all be made easier by being aware of the structure and function of the eye as well as the typical problems covered in this article. Many people may now discover efficient treatments for their eye conditions thanks to developments in medical science and technology, restoring their ability to see clearly and to their best ability.

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