Human Eye Defects:
In human beings, the normal eye is emmetropic. Thus, when a person looks at infinity, the rays incident to the eye are parallel which after refraction are focussed on the retina and form the image. But refractive errors of the eye are quite common in humans due to which the image of an object may be blurred, either formed in front of the retina or beyond the retina. Some common defects of the eyes are-
(1) Myopia (short-sightedness)- In this type of refractive error, parallel rays, i.e., rays coming from a distant object are focussed and the image is formed in front of the retina instead of on the retina. Therefore, the nearby objects are clearly visible but distant objects appear blurred to the person.
The common cause of this type of defect is an abnormally long eyeball or an abnormally high curvature of the lens.
Corrective Measure- Eyeglasses that are filled with concave lenses correct the eye’s error and bring the image of far-off objects into sharp focus on the retina.
(2) Hypermetropia (long-sightedness)- In this type of defect, parallel rays from the infinity are focussed behind the retina. Although distant objects can be seen clearly, the focal point of light from nearby objects is behind the retina. This makes it difficult for a far-sighted person to see the nearby objects clearly.
Corrective Measure- Use of convex lenses.
(3) Presbyopia- It is the far-sightedness of old age particularly above 40. It is similar to hypermetropia. It occurs due to the loss of elasticity of the lens in old age. It results in blurred near vision.
Corrective Measure- Use of convex lenses.
(4) Astigmatism- In this case cornea or lens is curved unequally in different directions. This results in a distorted image i.e., an image of some parts is formed in front and of some parts formed behind the retina.
Corrective Measure- Use of cylindrical lenses.
(5) Cataract- In this case lens becomes partially or completely opaque due to the ageing process. It is caused by the denaturation of lens proteins.
Corrective Measure- Surgical removal of the opaque lens and the normal vision is restored by the use of eyeglasses or contact lenses.
