Name the two major defects of an eye. Explain causes of their development and discuss methods of their correction.

Myopia (Near sightedness) and Hypermetropia (Far sightedness) are two major defects of eye.
Myopia arises because the power of the eye is too large due to the decrease in focal length of the lens which may be due to the excessive curvature of the cornea or elongation of eye ball.
This defect can be corrected by using concave (diverging) lens, which will bring the image of the object back on the retina itself.
Hypermetropia arises because of either the focal length of the eye lens is too large or the eyeball becomes too short, so that light rays from the nearby object cannot be brought to focus on the retina to give a distinct image.
This defect can be corrected by using a convex (converging) lens of appropriate focal length.