Defects of vision and their correction
Answers
Three common refractive defects of the eye:
Myopia or Near-sightedness
Hypermetropia or Farsightedness
Presbyopia
Myopia
Near-sightedness : A person with Myopia can see nearby objects clearly
A person with myopia cannot see faraway objects clearly.
The far point for the myopic eye is nearer than infinity
Occurs due to
Excessive curvature of the eye lens
Elongation of eyeball
The image of a distance object is formed in front of the retina and not on the retina
Defected is corrected by using Concave lenses such that the lens will bring the image back on to the retina.
Hypermetropia
Far sightedness : A person with Hypermetropia can see faraway objects clearly.
A person with Hypermetropia cannot see nearby objects clearly.
The near point of the eye is more than 25cm
This arises mostly during latter stages in life, as a result of the weakening of the ciliary muscles and/or the decreased flexibility of the lens.
Focal length of the eye lens is too long
Eyeball has become too small.
The image of a distance object is formed in behind the retina and not on the retina
Defected is corrected by using Convex lenses such that the lens will bring the image back on to the retina.
Presbyopia
The power of accommodation of the eye usually decreases with ageing. The ciliary muscles weaken and thereby the flexibility of the eye lens reduces.
The near point moves away.
Spectacles with convex lenses are recommended
Myopia and Hypermetropia
Sometimes a person may suffer from both near sightedness and far-sightedness.
Such people are advised to use bifocal lenses.
Bifocal lenses consists of concave on the upper portion and convex on the lower portion.
Concave supports distinct vision and convex supports near vision
Refractive eye defects can also be corrected using contact lenses or through specific surgical procedures.