Defects of Vision are conditions that cause the eye to not focus images properly on the retina, resulting in blurry or distorted vision. These defects can be corrected with the help of lenses or medical intervention.
1. Myopia (Nearsightedness)
Definition: Myopia is a condition where a person can see nearby objects clearly, but distant objects appear blurry. This occurs when the eye is too long or the cornea is too curved.
Cause:
- The image is focused in front of the retina instead of directly on it. This happens because the light rays entering the eye converge before reaching the retina.
Correction:
- Concave lenses (diverging lenses) are used to correct myopia. They diverge the incoming light rays, causing them to focus directly on the retina.
Diagram:
- In the case of myopia, a concave lens spreads the light rays that would have converged too early, helping them focus properly on the retina.
2. Hypermetropia (Farsightedness)
Definition: Hypermetropia is a condition where a person can see distant objects clearly, but nearby objects appear blurry. This occurs when the eye is too short or the cornea is too flat.
Cause:
- The image is focused behind the retina instead of directly on it. This happens because the light rays entering the eye converge too late.
Correction:
- Convex lenses (converging lenses) are used to correct hypermetropia. These lenses converge the light rays before they enter the eye, making them focus on the retina.
Diagram:
- In the case of hypermetropia, a convex lens helps focus light rays on the retina by converging them before they reach the eye.
3. Astigmatism
Definition: Astigmatism is a condition where the eye does not focus light evenly onto the retina, causing blurred or distorted vision at all distances. It is caused by an irregular shape of the cornea or lens, which can cause light to be focused at multiple points rather than a single point on the retina.
Cause:
- The cornea or lens is not spherical but has an irregular shape (e.g., more curved in one direction than another), which leads to different focal points for different orientations of light.
Correction:
- Cylindrical lenses (lenses with different curvatures in different directions) are used to correct astigmatism by compensating for the irregular shape of the eye.
Diagram:
- In the case of astigmatism, cylindrical lenses are used to correct the uneven curvature of the cornea and properly focus light on the retina.
4. Presbyopia
Definition: Presbyopia is an age-related condition where the ability to focus on close objects decreases, usually becoming noticeable after the age of 40. It occurs as the lens of the eye becomes less flexible and loses the ability to change shape for near vision.
Cause:
- The lens hardens and loses elasticity over time, making it difficult to focus on objects that are close to the eye.
Correction:
- Bifocal or multifocal lenses are used to correct presbyopia. These lenses have different focal points for distance and near vision. In some cases, reading glasses are used for near tasks.
Diagram:
- In presbyopia, bifocal lenses allow clear vision both for distant and nearby objects by using different regions of the lens for different distances.
| Defect | Cause | Correction |
|---|---|---|
| Myopia | Eye too long or cornea too curved. | Concave lenses (diverging lenses) |
| Hypermetropia | Eye too short or cornea too flat. | Convex lenses (converging lenses) |
| Astigmatism | Irregular shape of cornea or lens. | Cylindrical lenses |
| Presbyopia | Age-related loss of lens elasticity. | Bifocal or multifocal lenses, reading glasses |