The Human Eye and the Colourful World PDF
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Uploaded by JudiciousTroll
2024
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Summary
This document explains different types of eye defects such as myopia, hypermetropia, and presbyopia. It elaborates on their causes and potential corrections. The summary details how these defects affect the eye's ability to focus on near and far objects, and how suitable corrective lenses are used to remedy them. The main topics covered here are myopia, hypermetropia, and presbyopia.
Full Transcript
(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
