(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 eye ball or due to decrease of focal length.
Correction – Using concave lens.
After using concave lens, affected eye can see distant objects
(ii) Hypermetropia is long-sightedness, the image is formed behind the retina due to shortening of eyeball or due to increase in the focal length of the lens of eye.
Correction: Using convex lens.
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 ageing. 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.
