CBSE Class 10 Answered
Explain myopia, hypermetropia and astigmatism and their corrective measures
Asked by sehrawatam | 13 Dec, 2018, 06:15: PM
Expert Answer
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.
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 as shown in figure and not at the retina itself.
This defect may arise due to (i) excessive curvature of the eye lens, or (ii) elongation of the eyeball.
This defect can be corrected by using a concave lens of suitable power. This is illustrated in figure. A concave lens of suitable power
This defect can be corrected by using a concave lens of suitable power. This is illustrated in figure. A concave lens of suitable power
will bring the image back on to the retina and thus the defect is corrected.
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 as shown in figure.
from a closeby object are focussed at a point behind the retina as shown in figure.
This defect arises either because (i) the focal length of the eye lens is too long, or (ii) the eyeball has become too small.
This defect can be corrected by using a convex lens of appropriate power. This is illustrated in figure. Eye-glasses with converging lenses
provide the additional focussing power required for forming the image on the retina.
Astigmatism is an imperfection in the curvature of eye’s cornea or lens.
Normally, the cornea and lens are smooth and curved equally in all directions.
This helps to focus light rays sharply onto the retina at the back of eye.
If cornea or lens isn't smooth and evenly curved, light rays aren't refracted (bent) properly. Astigmatism is called as refractive error.
Answered by Thiyagarajan K | 14 Dec, 2018, 07:33: AM
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