Human eye and the colorful world Class 10 Notes | EduRev

Class 10 : Human eye and the colorful world Class 10 Notes | EduRev

 Page 2


 
 
PHYSICS THE HUMAN EYE AND THE COLOURFUL WORLD 
 
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Defects of Vision 
Defects of Vision and their Correction 
? There are three refractive defects of vision caused in the human eye. 
 
 
 
Myopia or 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 up to a 
distance of a few metres. 
 
 
? In a myopic eye, the image of a distant object is formed in front of the retina 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. A concave lens of suitable 
power will bring the image back onto the retina and thus the defect is corrected. 
 
 
Myopia or near-sightedness
Hypermetropia or far-sightedness
Presbyopia
Page 3


 
 
PHYSICS THE HUMAN EYE AND THE COLOURFUL WORLD 
 
  www.topperlearning.com  2 
Defects of Vision 
Defects of Vision and their Correction 
? There are three refractive defects of vision caused in the human eye. 
 
 
 
Myopia or 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 up to a 
distance of a few metres. 
 
 
? In a myopic eye, the image of a distant object is formed in front of the retina 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. A concave lens of suitable 
power will bring the image back onto the retina and thus the defect is corrected. 
 
 
Myopia or near-sightedness
Hypermetropia or far-sightedness
Presbyopia
 
PHYSICS THE HUMAN EYE AND THE COLOURFUL WORLD 
 
  www.topperlearning.com  3 
Hypermetropia or Far-Sightedness 
? A person with hypermetropia can see distant objects clearly but cannot see nearby objects distinctly. 
? A person with this defect has the near point farther away from normal near point of 25 cm. Such a 
person has to keep the reading material farther than 25 cm. 
 
 
? In this case, the image of a nearby object is focused behind the retina.  
 
 
 
? This defect may arise because (i) 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 suitable power. Eye glasses with converging 
lenses provide the additional focusing power required for forming the image on the retina. 
 
 
 
Presbyopia 
? With age, the power of accommodation decreases. For most people, the near point gradually recedes 
farther away. This defect is called presbyopia. 
? It arises due to gradual weakening of the ciliary muscles and reducing flexibility of the eye lens. 
? Sometimes, a person may suffer from both myopia and hypermetropia. Such people require a bi-focal 
lens. 
? The upper part of a bi-focal lens consists of concave lens facilitating distant vision, and the lower part 
consists of convex lens facilitating nearby vision. 
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