Far Sightedness Hypermetropia


 
 
Concept Explanation
 

Far Sightedness Hypermetropia

Far-sightedness (Hypermetropia): Some people cannot see nearby objects clearly, although they can see distant objects clearly. This defect is called far-sightedness, long- sightedness, hypermetropia or hyperopia. It arises mainly due to two reason

  • In some people, the eyeball becomes shorter than the normal. As a result, the distance of the retina from the crystalline lens decreases. Then, minimum focal length of the eye-lens, i.e., when the ciliary muscles contracted, the image of a nearby object is formed behind the retina as in figure (a). Thus, nearby objects cannot be seen clearly.
  • This defect can also occur if the eye-lens has a focal length that is larger than the normal due to abnormal curvatures of the cornea or the crystalline lens.
  • People suffering from far-sightedness have difficulty in reading a book or a newspaper, or viewing a small object placed close to the normal near point, N (at 25 cm). People suffering from far-sightedness have to keep reading material or an object much farther away to see clearly. In other words, their near point, N', is farther than the normal near point, N

    Correcting far-sightedness: To see an object placed at the normal near point, N, clearly, its sharp image should be formed on the retina. For this, the rays should seem to be coming from N', the near point of the defective eye. To do this, the rays are made less divergent by placing a convex lens in front of the eye as in figure (c). Rays of light coming from N pass through the convex lens. When produced backwards, the emergent rays meet at the point N'. Thus, a sharp image is formed on the retina.

     

     

     

    Sample Questions
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    Question : 1

    Which of the following are correct :

    (a) Far-sightedness is also known as long- sightedness, hypermetropia or hyperopia.

    (b) In farsightedness, people cannot see nearby objects clearly, although they can see distant objects clearly.

    Right Option : C
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    Question : 2

    Which of the following are the reason of farsightedness :

    (a) The eyeball becomes shorter than the normal. As a result, the distance of the retina from the crystalline lens decreases.

    (b) The eyeball becomes wider than the normal. As a result, the distance of the retina from the crystalline lens decreases.

    (c) If the eye-lens has a focal length that is larger than the normal due to abnormal curvatures of the cornea.

    (d) if the eye-lens has a focal length that is smaller than the normal due to abnormal curvatures of the cornea.

    Right Option : C
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    Question : 3

    A person cannot see objects clearly which are nearer than 75cms from his eyes, the disease he is suffering from is _________________.

    Right Option : C
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