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Human Eye and Colourful World — Science STD 10 — Question

CBSE BoardEnglish MediumSTD 10ScienceHuman Eye and Colourful World5 Marks
Question
  1. List three common refractive defects of vision. Suggest the way of correcting these defects.
  2. About 45 lac people in the developing countries are suffering from corneal blindness. About 30 lac children below the age of 12 years suffering from this defect can be cured by replacing the defective cornea with the cornea of a donated eye. How and why can students of your age involve themselves to create awareness about this fact among people?
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Answer

  1. The three defects of vision:
  1. Myopia: Myopia is the defect of the eye vision due to which a person can see the near objects clearly, but he can not see the far objects clearly. Causes of myopia. Myopia is caused due the elongation of the eye ball and due to decrease in the focal length of the eye lens. The eye lens become more convergent. Myopia can corrected by using a concave lens of suitable focal length in the spectacles of such a person.
  1. Long-sightedness or hypermetropia: Due to defect, a person is not able to see the nearby objects clearly but can see the distant objects clearly.
Cause of hypermetropia. It is caused due to the following reasons:
  • Normal increase in the focal length of the eye lens. The lens becomes less convergent.
  • Shortening of the eyeball size.
correction. Long sightedness can be corrected by using a convex lens of suitable focal length in the spectacles of such a person.
When a convex lens of suitable power is placed in front of the hypermetropic eye then diverging rays of light coming from the nearby object are the first converged by this convex lens. Due to this, the convex lens from a virtual image of the nearby object at a point near to the hypermetropic eye. Then the hypermetropic eye can easily focus the image formed by convex lens on the retina.
  1. Presbyopia: The power of accommodation of the eye decreases with ageing. It occurs due to the gradual weakening of the ciliary muscles and decreasing flexibility of the crystalline lens. The near point of the eye increases with age. It may reach even 2 metres. This defect is called presbyopia.
Causes of presbyopia. It is mainly caused due to the weakening of the ciliary muscles of the eyes. Such a person may suffer from myopia and hypermetropia. This defect is then corrected by using bifocal lenses of suitable focal lengths. The upper part of the lens is a concave lens for correcting myopia to see the distant objects clearly, while the lower part of the lens has a convex lens to correct the hypermitropia to see the nearby objects clearly.
  1. We have seen advertisements in media persuading people for eye donation. If cornea of the eyes are removed within 6 hours of the death of a person, it can be transplanted to a person suffering from corneal blindness. There are more than 45 lakh cases of corneal blindness and unfortunately out of these a major portion is that of children aged below 12 years. If a person comes forward for eye donation, he can save two corneal blind persons by donating one eye to each.
How exciting it feels to think that after one’s death, he/she can make two blind persons see this wonderful world Therefore it is a must to promote such advertisements to encourage people and make them aware of this noble cause.

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