VISION DEFECTS

How normal eye works? In the case of a normal eye, the rays of light from the object fall on the eye and converge on the retina 

Vision Defects It is the impairment of the sense of sight . Any abnormality of the eye that causes less-than-perfect sight. Two most common defects are myopia or nearsightedness and hypermetropia or farsightedness.

Myopia or Nearsightedness distant objects appear out of focus, while objects that are within close range can be seen clearly.

It is a condition where the eye is too long in relation to its refractive power. The light rays therefore intersect in front of the retina instead of on it. Blurred vision results. The further away the object observed, the more blurred it appears. Clear vision is limited to nearby objects. 

Causes of Myopia the elongation of the eye ball, that is, the distance between the retina and eye lens is increased decrease in focal length of the eye lens

A myopic eye forms the image of a far off object in front of the retina because of the increase in converging power of the eye lens. Therefore myopia can be rectified by using a suitable divergent or concave lens. The ray diagram shows how a concave lens helps in focusing the light on the retina.

Hypermetropia or Farsightedness distant objects can be seen clearly, while objects that are within close range appear out of focus.

in the case of a hypermetropic eye, the light rays are focused behind the retina.

Causes of Hypermetropia Shortening of the eyeball, that is, the eyeball becomes smaller Increase in focal length of the eye lens

long sightedness is due to the decreased converging power of the lens  long sightedness is due to the decreased converging power of the lens. Therefore hypermetropia can be rectified by making the eye lens more convergent. This is done by placing a convex lens of suitable focal length before the eye lens as shown in the figure.

Astigmatism It is another common vision defect in which eyesight may be blurred at all distances. People with minor astigmatism may believe to have a clear vision, but find they are squinting from time to time or suffering from headaches, eye strain, fatigue and blurred vision depending on.

Causes of Astigmatism Normally the cornea is spherical in shape but in this case, cornea is shaped more like an oblong Rugby ball

Astigmatism’s Test Close one eye and then the other one , if you do not see all the lined squares, in the same black color , if you do see one or more squares grey, you than have an astigmatism.

When light rays hit the steep sides of the cornea, they are focused at two points instead of at one point on the retina (the nerve-rich lining inside the eye, where we “see”), producing blurred or distorted images. 

This can be corrected by using cylindrical or asymmetric lenses.

Color Blindness is another common vision defect in which colors are not seen normally. Most color-blind people find it difficult to see or distinguish greens from reds and vice versa. Some may not recognize any colors at all. 

Causes of Color Blindness The various degrees of color blindness are caused by the state of the color receptors found at the back of the eye. 

While color blindness cannot be cured, color-blind people can develop personalized color identification systems for their everyday needs, or learn to recognize colors by their brightness or location. Visual aids may also be used to help color-blind people distinguish colors to some extent. 

Cataract   a clouding of the lens in the eye that affects vision. Most cataracts are related to aging. Cataracts are very common in older people. 

Types of Cataract Secondary cataract. Cataracts can form after surgery for other eye problems, such as glaucoma. Cataracts also can develop in people who have other health problems, such as diabetes. Cataracts are sometimes linked to steroid use. Traumatic cataract. Cataracts can develop after an eye injury, sometimes years later. Congenital cataract. Some babies are born with cataracts or develop them in childhood, often in both eyes. These cataracts may be so small that they do not affect vision. If they do, the lenses may need to be removed. Radiation cataract. Cataracts can develop after exposure to some types of radiation.

Causes of Cataract as we age, some of the protein may clump together and start to cloud a small area of the lens. This is a cataract. Over time, the cataract may grow larger and cloud more of the lens, making it harder to see. Researchers suspect that there are several causes of cataract, such as smoking and diabetes. Or, it may be that the protein in the lens just changes from the wear and tear it takes over the years.

How Cataract can be treated? The symptoms of early cataract may be improved with new eyeglasses, brighter lighting, anti-glare sunglasses, or magnifying lenses. If these measures do not help, surgery is the only effective treatment. Surgery involves removing the cloudy lens and replacing it with an artificial lens.

Presbyopia  is a common vision condition in which it becomes increasingly difficult over time to focus on objects that are within close range. 

Causes of Prebyopia  is caused when the centre of the eye lens hardens making it unable to accommodate near vision. This condition generally affects almost everyone over the age of 50 - even those with myopia. 

 Eyeglasses with bifocal or progressive lenses are prescribed to correct or improve the condition. The contact lenses used by presbyopes are multifocal lenses.

How are color vision defects diagnosed? Specialized color vision tests can easily detect color vision defects. Pseudoisochromatic plate tests are commonly used to screen for inherited color vision defects. In this group of tests, a pattern of colored dots forms a number or letter against a background of other colored dots. Persons with normal color vision can discern these patterns but those with color defects cannot.

Pseudoisochromatic Plate Test

A Snellen chart is an eye chart used by eye care professional and others to measure visual acuity. The normal height for the letter G is 88 mm, and the viewing distance is 6 meters.

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