Defects of vision and its correction Flashcards
what is cataract? how can it be cured?
sometimes, the crystalline lens of people at old age becomes milky or cloudy, this condition si called CATARACT. This leads to the partial or complete loss of vision. It is possible to restore vision through a cataract surgery. In a cataract surgery, the cloudy lens is removed and an artificial lens is implanted
what causes eye defects
sometimes, the eye may gradually loses its power of accomodation. In such conditions, the person cannot see the objects distinctly and comfrotably. The vision becomes blurred due to some refractive defects of the eye.
describe myopia
Myopia is also known as near-sightedness. A person with myopia can see nearby objects clearly but cannot see distant objects distinctly. A person with this defect has the far point nearer than infinity. Such a person may see clearly upto a distance of a few metres. In a myopic eye, the image of a distant object is formed in front of the retina [Fig. 11.2 (b)] and not at the retina itself.
what causes myopia
this defect of myopia is caused either by
i) excessive curvature of the eyelens
or
ii)elongation of the eyeball
how can myopia be corrected
this defect can be corrected by using a concave lens of suitable power, this will bring back the image on the retina, and the defect will be corrected
describe hypoermetripoa
Hypermetropia is also known as far-sightedness. A
person with hypermetropia can see distant objects
clearly but cannot see nearby objects distinctly. The
near point, for the person, is farther away from the
normal near point (25 cm). Such a person has to keep
a reading material much beyond 25 cm from the eye
for comfortable reading. This is because the light rays
from a closeby object are focussed at a point behind
the retina
what causes hypermetropia
this defect because either
i)the focal length of the eye lens is too long
or
ii)the eyeball has become too small
how can hypermetropia be correctedn
this defect can be corrected with convex lens of suitable power. eyeglasses with converging lens provide the additional focussing power required to form the image on the retina.
describe about presbyopia
The power of accommodation of the eye usually
decreases with ageing. For most people, the near point
gradually recedes away. They find it difficult to see
nearby objects comfortably and distinctly without
corrective eye-glasses. This defect is called Presbyopia.
It arises due to the gradual weakening of the ciliary
muscles and diminishing flexibility of the eye lens.
how is presbyopia corrected
Sometimes, a person may suffer from both myopia and
hypermetropia. Such people often require bi-focal lenses. A common
type of bi-focal lenses consists of both concave and convex lenses.
The upper portion consists of a concave lens. It facilitates distant
vision. The lower part is a convex lens. It facilitates near vision
what are new soln’s for refractive defects
These days, it is possible to correct the refractive defects with
contact lenses or through surgical interventions.
what causes cataract
- due to old age the crystalline becomes cloudy and milky
- the uv radiation from the sun causes the lens to be cloudy/milky
describe abt astigmatism
Astigmatism is an optical defect in which vision is blurred due to the inability of the optics of the eye to focus a point object into a sharp focused image on the retina. This may be due to an irregular curvature of the cornea or lens.
