Wednesday, February 16, 2022

11. The Human Eye and the Colourful World | Class 10 CBSE | Web Notes | Part 2: Defects of Vision and their Correction

THE HUMAN EYE AND THE COLOURFUL WORLD


DEFECTS OF VISION AND THEIR CORRECTION


Defects of vision (refractive defects) are caused due to gradual loss of power of accommodation.


Mainly 3 types: Myopia, Hypermetropia & Presbyopia.


(a) Myopia (near-sightedness)


Here, a person can see nearby objects clearly but cannot see distant objects distinctly.


A myopic person has the far point nearer than infinity.


This is caused due to the formation of an image of a distant object in front of the retina.


Reasons:

(i)       Excessive curvature of the lens.

(ii)     Elongation of the eyeball.


Myopia can be corrected by using a concave lens of suitable power. It brings the image back on to the retina.


(b) Hypermetropia (far-sightedness)


Here, a person can see distant objects clearly but cannot see nearby objects distinctly.


The near point is farther away from the normal near point (25 cm). Such a person has to keep a reading material beyond 25 cm from the eye.


This is because the light rays from a close object are focussed at a point behind the retina.


Reasons:

(i)    Focal length of the eye lens is too long.

(ii)  Eyeball becomes too small.


This can be corrected by using convex lens (converging lenses) of suitable power. It provides additional focussing required to form the image on the retina.




(c) Presbyopia


Here, the power of accommodation of the eye decreases with ageing. For most people, the near point gradually recedes away. So comfortable and distinct vision of nearby objects is not possible.


It is due to the gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens.


Some people may have both myopia & hypermetropia. They require bi-focal lenses. Its upper part is a concave lens for distant vision. The lower part is a convex lens for near vision.


Refractive defects can also be corrected with contact lenses or through surgery.

About 35 million people in the developing world are blind.

About 4.5 million people with corneal blindness can be cured through corneal transplantation. Of these, 60% are children below the age of 12. So, eye donation is important.

·   Eye donors can belong to any age group or sex.

·   Eyes must be removed within 4-6 hours after death.

·   The eye bank team removes the eyes at the home of the deceased or at a hospital. It takes only 10-15 minutes without causing any disfigurement.

·   Persons who were infected with or died because of AIDS, Hepatitis B or C, rabies, acute leukaemia, tetanus, cholera, meningitis or encephalitis cannot donate eyes.

The donated eyes unsuitable for transplantation are used for research & medical education. One pair of eyes gives vision to up to 4 corneal blind people.


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