Myopia-The eye defect in which a person is not able to see distant objects clearly is called myopia. This defect can be corrected by using a concave lens. In this type of defect, the image of a distant object is formed in front of retina.
this defect arises because the power of the eye is too great due to the decrease in focal length of the crystalline lens. This may arise due to the either:
(i)excessive curvature of the cornea or
(ii)eye ball being too long.
hypermetropia-In this case the image of the nearby objects is formed behind the objects. Thus nearby objects cannot be seen clearly, although faraway objects can, this defect is called far-nearsightedness, foresightedness, hypermetropia. This defect can be corrected by using a convex lens of appropriate focal length.
This defect may arise because either:
(i)the focal length of the eyeball is too large or
(ii)the eye ball becomes too short
presbyopia-With growing age, the power of accommodation of the eye decreases. Due to this, most people cannot read comfortably and distinctly, without having eyeglasses. This defect is called is called Presbyopia. This defect arises due to the gradual weakening of the ciliary muscles and diminishing flexibility of the crystalline lens. When a person suffers from both myopia and hypermetropia, often requires a bifocal lens.