Difference between Myopia and hypermetropia
Myopia
1) The defect in human eye in which one can see nearby objects clearly, but cannot see distinct object clearly is known as myopia or near sightedness.
2) In Myopia, ciliary muscles expands the lens, but doesn’t relaxed, so that the focal length of eye lens decreases, eyeball gets elongated.
3) Image of object forms before retina due to convergence of rays before retina since the focal length is decreased.
4) This makes person able to see nearby objects clearly but can’t make him/her able to see distinct objects.
5) To correct this defect, an ophthalmic doctor prescribes the concave lens of suitable power (or focal length) to use in spectacles.
6) Hence the power of lens for the person with myopia is negative.
Hypermetropia
1) The defect in human eye in which one can see distinct objects clearly, but cannot see nearby object clearly is known as hypermetropia or far sightedness.
2) In Hypermetropia, ciliary muscles contracts the lens, but doesn’t relaxed, so that the focal length of eye lens increases, eyeball gets flattered.
3) Image of object forms behind retina due to divergence of rays after retina since the focal length is increased.
4) This makes person able to see distinct objects clearly but can’t make him/her able to see nearby objects.
5) To correct this defect, an ophthalmic doctor prescribes the convex lens of suitable power (or focal length) to use in spectacles.
6) Hence the power of lens for the person with hypermetropia is positive.