What Do We Understand By Eye Defects?
Vision is one of the most precious gifts that has been given to us by nature. The human eye is one of the five senses possessed by human beings and it is a part of the sensory nervous system, the eye reacts to light and helps humans achieve their vision. There are various accessory organs in the human eye, like the pupil, lens, retina, cornea, iris, etc. All the accessory organs of the eye work in synergy for humans to achieve their sense of vision. The human eye can see both nearby and distant objects but the working mechanism is different in both cases. When we are looking at distant objects, the rays diverge and become parallel when they reach the eye, so to see distant objects, the concave eye lens is required because it has a large focal length, and it's thick too. But when we are looking at nearby objects, the object rays diverge when it reaches the eye. In this case, the convex eye lens of high converging power and short focal length is required.
Most Common Eye Defects
Eye defects are a major health hazard in these times, it can be defined as the disability of ciliary muscles, these ciliary muscles are pivotal for the changing of the shape of the eye lens. The disability of the ciliary muscles can cause problems regarding image formation of any object in the retina and can lead to the defect of vision. The three most common types of eye defects seen in humans are:
Nearsightedness or Myopia – In which distant objects are not seen clearly.
Farsightedness or Hypermetropia – In which Nearby objects are not seen clearly.
Presbyopia – It is generally seen in elderly age groups in which distant as well as nearby objects, both are not seen clearly.
What is Myopia?
When a person is not able to see distant objects clearly until the object is kept closer to the eye, it is stated as myopia. Myopia is also called nearsightedness or short-sightedness. All the nearer objects are clearly visible to myopic patients. Two issues usually cause myopia; they are:
High converging power of the lens
Eye-ball elongation
When the image is formed in front of the retina, the person is not able to see distant objects clearly, and this happens because of the high convergence of the eye lens.
And in the second case, that is when the eyeball gets too elongated, the retina then gets distant from the eye-lens. Over here, the eye lens has good converging power, but then also the image gets formed in front of the retina.
Because of myopia, the person gets blurry vision while driving. This defect can develop in childhood and can grow gradually or rapidly depending upon the power of the eye.
This eye defect is overgrowing, and the most common symptoms are eye strain, fatigue, headache, and squinting. Myopia usually occurs when a person reads a book or computer closely. This can even happen to any individual who spends less time outdoors.
How is Myopia Corrected?
If a person has nearsightedness or short-sightedness, then they are asked to wear spectacles, including a concave lens.
A concave lens is used to treat myopic eyes because the lens diverges the parallel light rays coming from distant objects. Hence the virtual image is formed at a distant point of the myopic eye.
And now, as the light rays are at the proper distance, the objects can be easily focused by the lens, and the image is thus formed on the retina. In simpler words, the concave lens decreases the converging power.
A myopic eye can also be treated by a refractive survey or by contact lenses. Although refractive surgeries are costly, it gives good results in the end. There are different degrees of myopia.
The formula for calculating the power of the concave lens for correcting myopia is:
1/image distance (v) - 1/object distance (u) = 1/focal length (f) i.e., (v-u)/f.
Myopia Defect and Blindness
Myopia is a severe defect and can aggravate under extreme circumstances. There is a stage when myopia is at its peak, and this stage is called degenerative nearsightedness or high myopia. Although this stage is quite rare, once reached at this stage, it becomes impossible for a person to retain normalcy, and they can lose their eyesight.
The most serious complication seen in the myopic patient is cataract, retinal detachment, and glaucoma. The seriousness can be further divided into: pathological myopia, and degenerative myopia. Among these three issues, the one which can lead to blindness is retinal detachment.
Conclusion:
The Human eye is a very sensitive organ, it has a very advanced mechanism where the human eye and its organs work in tandem with the nerves of the human mind to help with the process of image formation. There should be proper care taken of the eyes as it is the most significant sense which is necessary for our daily functioning. People who use phones and laptops for a long duration of time should use blue screens and spectacles which protect them from radiation.
FAQs on Eye Defects and Correction
1. What are the common defects of vision as per the CBSE syllabus?
The human eye can suffer from refractive defects of vision, where it loses its ability to focus light correctly on the retina. The three common defects studied are:
- Myopia (Nearsightedness): A person can see nearby objects clearly but distant objects appear blurry. This occurs because the image is formed in front of the retina.
- Hypermetropia (Farsightedness): A person can see distant objects clearly but finds it difficult to see nearby objects. This is because the image is formed behind the retina.
- Presbyopia: This is the gradual loss of the eye's ability to focus on nearby objects, typically occurring in old age. It is caused by the weakening of the ciliary muscles and diminishing flexibility of the eye lens.
2. How are the primary vision defects like myopia and hypermetropia corrected?
Vision defects are corrected by using spectacles with appropriate lenses that adjust the focal length so the image forms correctly on the retina.
- Myopia is corrected using a concave lens of a suitable power. This diverging lens pushes the focal point backward onto the retina.
- Hypermetropia is corrected using a convex lens of a suitable power. This converging lens brings the focal point forward onto the retina.
- Presbyopia is often corrected using bifocal lenses, which have both concave (for distant vision) and convex (for near vision) parts.
3. What is the difference between hypermetropia and presbyopia?
While both hypermetropia and presbyopia cause difficulty in seeing near objects, their underlying causes are different. Hypermetropia is typically caused by the eyeball being too short or the eye lens having a focal length that is too long. In contrast, presbyopia is an age-related condition caused by the weakening of the ciliary muscles and a decrease in the flexibility of the eye lens, reducing its power of accommodation. An individual can have hypermetropia from a young age, whereas presbyopia develops during old age.
4. What causes refractive eye defects like myopia and hypermetropia to occur?
Refractive eye defects are primarily caused by physical changes in the eye's structure that affect how it focuses light. The two main causes are:
- Change in Eyeball Length: If the eyeball becomes too long from front to back, light focuses in front of the retina, causing myopia. If the eyeball is too short, light focuses behind the retina, causing hypermetropia.
- Change in Lens Curvature: If the cornea or eye lens has an excessive curvature, it converges light too strongly, leading to myopia. Conversely, if the lens has insufficient curvature, it doesn't bend light enough, leading to hypermetropia.
5. Why exactly is a concave lens used for myopia and a convex lens for hypermetropia?
The choice of lens is based on correcting the path of light rays entering the eye. In myopia, the eye's lens converges light too much, forming an image *in front* of the retina. A concave (diverging) lens is used to spread the light rays out slightly before they enter the eye. This effectively moves the final focal point backward, so it lands directly on the retina. In hypermetropia, the eye's lens doesn't converge light enough, and the image would form *behind* the retina. A convex (converging) lens is used to bend the light rays more, moving the focal point forward onto the retina.
6. What is the power of accommodation of the eye, and how does its failure lead to defects?
The power of accommodation is the ability of the eye lens to adjust its focal length to see both distant and nearby objects clearly. This is achieved by the action of the ciliary muscles, which can change the curvature of the flexible eye lens. When this ability diminishes, typically due to ageing, the eye can no longer focus on near objects effectively. This specific failure of the power of accommodation results in the vision defect known as presbyopia.
7. How does the 'power' of a corrective lens, measured in dioptres, relate to an eye defect?
The power of a lens, measured in dioptres (D), indicates its ability to converge or diverge light. A higher dioptre value signifies a stronger lens. For eye defects, this power is directly related to the severity of the condition. For example, a person with severe myopia might need a -5.0 D lens, while someone with mild myopia might only need a -1.0 D lens. The negative sign indicates a concave lens (for myopia), and a positive sign indicates a convex lens (for hypermetropia). The power required is calculated to precisely shift the focal point onto the retina.
8. Is it possible for a person to have different vision defects in each eye?
Yes, it is possible. This condition is known as anisometropia, where the two eyes have different refractive powers. For instance, one eye could be myopic (nearsighted) while the other is hypermetropic (farsighted), or both could be myopic but to different degrees. This can cause issues like unequal image sizes, leading to headaches or dizziness. It is corrected by using spectacles where each lens is prescribed with a different power specific to that eye's needs.
9. What is astigmatism and how is it corrected?
Astigmatism is a common vision defect caused by an imperfectly shaped cornea or lens, which has an irregular, more 'oval' shape instead of a perfect sphere. This irregularity causes light to focus on multiple points instead of a single point on the retina, resulting in distorted or blurry vision at all distances. Astigmatism is corrected using a special type of lens called a cylindrical lens (or a toric lens), which is designed to compensate for the specific, directional imperfection in the eye's curvature.
10. Can eye defects like myopia be reversed or improved by eye exercises or lifestyle changes?
Currently, there is no scientific evidence to suggest that eye exercises can reverse refractive errors like myopia or hypermetropia, as these are caused by the physical shape of the eyeball and lens. However, certain lifestyle habits can influence their progression. Reducing prolonged near-work, taking frequent breaks from screens (the 20-20-20 rule), and spending time outdoors may help slow the progression of myopia in children and reduce eye strain in adults. The standard and effective methods for correction remain spectacles, contact lenses, or refractive surgery.
