What are hypermetropia and myopia? What are their causes? How are they rectified?

(N/A) These are common defects of vision.
Hypermetropia (Farsightedness): $A$ person suffering from this defect can see distant objects clearly but cannot see nearby objects clearly. In this defect,the near point of the eye shifts farther away than $25 \ cm$.
Causes:
$1.$ The eyeball becomes too short.
$2.$ The focal length of the eye lens becomes too large.
Correction: This condition is corrected using a converging or convex lens.
Myopia (Nearsightedness): $A$ person suffering from myopia can see nearby objects clearly but cannot see distant objects clearly. In this defect,the far point of the eye comes closer than infinity.
Causes:
$1.$ The eyeball becomes too long.
$2.$ The focal length of the eye lens becomes too small.
Correction: This condition is corrected using a diverging or concave lens.