Explanation:

The inability to see the distant objects clearly and distinctly is called short sightedness or Myopia. This defect arises when the image is formed infront of the retina. A short sighted person can see near objects clearly. This may arise due to either excessive curvature of the cornea or elongation of the eyeball. This defect is corrected by wearing glasses with a concave lens.
The inability to see near objects clearly and distinctly is called long sightedness or Hypermetropia. This defect arises when the image is formed behind the retina. This defect may arise due to shortening of eye ball. A long sighted person can see the distant objects clearly. This defect is corrected by wearing spectacles with convex lens (converging) of appropriate focal length. A converging lens will correct this defect by converging the incoming rays so that the image is formed on the retina.