Name three refractive defects of vision with help of diagram. Explain its reasons and correction of these defects. ?
The three refractive defects of vision are (i) Myopia (ii) Hypermetropia (iii) Presbyopia (i) Myopia is short-sightedness, the image is formed in front of retina due to the elongation of the eyeball or due to decrease of focal length. Correction - Using a concave lens. After using a concave lens, the affected eye can see distant objects (ii) Hypermetropia is long-sightedness, the image is formed behind the retina due to shortening of the eyeball or due to an increase in the focal length of the lens of the eye. Correction: Using convex lens. (c) After using convex lens, the affected eye can see nearby objects (iii) Presbyopia: It is the defect of an eye in which the power of accommodation of the eye usually decreases with aging. Near point changes as well as the far off objects are also not visible clearly. It is caused due to the weakening of ciliary muscles and the reduced flexibility of eye lens. Such a defect in which a person suffers from both myopia and hypermetropia is called presbyopia. It is corrected by using bifocal lens.
