CBSE · Class 10 · Science · The Human Eye and Colourful WorldWhat are the three common defects of vision? Explain each defect with its causes and corrective measures using suitable diagrams or descriptions.

Common Defects of Vision and Their Correction

\nDue to the aging of ciliary muscles or changes in the shape of the eyeball, the human eye may lose its ability to accommodate. This leads to refractive defects of vision. The three common refractive defects are:

1. Myopia (Near-sightedness)

• Meaning: A person with myopia can see nearby objects clearly, but cannot see distant objects distinctly. The far point for such a person is nearer than infinity.
• Causes:
  • Excessive curvature of the eye lens.
  • Elongation of the eyeball.
• Correction: The image of a distant object is focused in front of the retina. This defect is corrected by using a concave lens of suitable power, which diverges the incoming light rays so that they are focused on the retina.

2. Hypermetropia (Far-sightedness)

• Meaning: A person with hypermetropia can see distant objects clearly, but cannot see nearby objects distinctly. The near point for such a person is greater than 25 cm.
• Causes:
  • Focal length of the eye lens is too long.
  • Eyeball has become too small.
• Correction: The image of a nearby object is focused behind the retina. This defect is corrected by using a convex lens of suitable power, which converges the light rays before they enter the eye.

3. Presbyopia

• Meaning: It is the defect of vision arising due to aging, where a person finds it difficult to see nearby objects comfortably without corrective glasses. It is a type of old-age hypermetropia.
• Causes:
  • Gradual weakening of the ciliary muscles.
  • Diminishing flexibility of the crystalline lens.
• Correction: This defect can be corrected by using a bifocal lens (consisting of both concave and convex lenses, where the upper part consists of a concave lens for distant vision and the lower part consists of a convex lens for reading purposes).