List four common refraction defects of vision. Suggest the way of correcting these defects.
Defects of vision: (i) Cataract: Crystalline lens of people at old age becomes milky and cloudy. This condition is called cataract. It is possible to restore vision through cataract surgery. (ii) Myopia: (Near sightedness) A person with myopia can see nearby objects clearly but cannot see distant objects clearly. Cause: Due to excessive curvature of the eye lens. Elongation of the eyeball. (iii) Hypermetropia (far-sightedness) A person with hypermetropia can see distant objects clearly but cannot see nearby objects distinctly. Cause: The focal length of the eye lens is too long. The eyeball has become too small. (iv) Presbyopia The power of accommodation of the eye usually decreases with ageing. In this eye defect it is difficult to see nearby objects comfortably and distinctly without corrective eye glasses. Cause: Weakening of ciliary muscles and diminishing flexibility of eye lens.
