Human Eye and Colourful World PDF

(a) Myopia Myopia is also known as near - sightedness. A person with myopia can see nearby objects clearly but cannot see distant objects distinctly. A person with this defect has the far point nearer than infinity. Such a person may see clearly upto a distanc...

(a) Myopia Myopia is also known as near - sightedness. A person with myopia can see nearby objects clearly but cannot see distant objects distinctly. A person with this defect has the far point nearer than infinity. Such a person may see clearly upto a distance of a few metres. In a myopic eye, the image of a distant object is formed in front of the retina [Fig. 10.2 (b)] and not at the retina itself. This defect may arise due to (i) excessive curvature of the eye lens, or (ii) elongation of the eyeball. This defect can be corrected by using a concave lens of suitable power. This Figure 10.2 is illustrated in Fig. 10.2 (c). A (a), (b) The myopic eye, and (c) correction for myopia with a concave lens of suitable power will concave lens bring the image back on to the retina and thus the defect is corrected. (b) Hypermetropia Hypermetropia is also known as far-sightedness. A person with hypermetropia can see distant objects clearly but cannot see nearby objects distinctly. The near point, for the person, is farther away from the normal near point (25 cm). Such a person has to keep a reading material much beyond 25 cm from the eye for comfortable reading. This is because the light rays from a closeby object are focussed at a point behind the retina as shown in Fig. 10.3 (b). This defect arises either because (i) the focal length of the eye lens is too long, or (ii) the eyeball has become too small. This defect can be corrected by using a convex lens of appropriate power. This is illustrated in Fig. 10.3 (c). Eye-glasses with converging lenses provide the additional focussing power required for forming the image on the retina. (c) Presbyopia Figure 10.3 The power of accommodation of the eye usually (a), (b) The hypermetropic eye, and (c) decreases with ageing. For most people, the near correction for hypermetropia point gradually recedes away. They find it difficult N = Near point of a to see nearby objects comfortably and distinctly hypermetropic eye. without corrective eye-glasses. This defect is N’ = Near point of a called Presbyopia. It arises due to the gradual normal eye. The Human Eye and the Colourful World 163 2024-25  weakening of the ciliary muscles and diminishing flexibility of the eye lens. Sometimes, a person may suffer from both myopia and hypermetropia. Such people often require bi-focal lenses. A common type of bi-focal lenses consists of both concave and convex lenses. The upper portion consists of a concave lens. It facilitates distant vision. The lower part is a convex lens. It facilitates near vision. These days, it is possible to correct the refractive defects with contact lenses or through surgical interventions. Q U E S T I O N S 1. What is meant by power of accommodation of the eye? 2. A person with a myopic eye cannot see objects beyond 1.2 m distinctly. ? What should be the type of the corrective lens used to restore proper vision? 3. What is the far point and near point of the human eye with normal vision? 4. A student has difficulty reading the blackboard while sitting in the last row. What could be the defect the child is suffering from? How can it be corrected? Think it over You talk of wondrous things you see, You say the sun shines bright; I feel him warm, but how can he Or make it day or night? – C. CIBBER Do you know that our eyes can live even after our death? By donating our eyes after we die, we can light the life of a blind person. About 35 million people in the developing world are blind and most of them can be cured. About 4.5 million people with corneal blindness can be cured through corneal transplantation of donated eyes. Out of these 4.5 million, 60% are children below the age of 12. So, if we have got the gift of vision, why not pass it on to somebody who does not have it? What do we have to keep in mind when eyes have to be donated? n Eye donors can belong to any age group or sex. People who use spectacles, or those operated for cataract, can still donate the eyes. People who are diabetic, have hypertension, asthma patients and those without communicable diseases can also donate eyes. 164 Science 2024-25