EXPLAIN : DEFECTS OF VISION
ANS:
We can see an object clearly when its image is formed exactly on retina.
The thickness of the lens of the eye changes according to the distance of the object.
Ciliary muscles help the lens to change its thickness.
When the lens can focus the object properly its image is formed exactly on retina and we can see it clearly.
When the lens cannot change its thickness properly, the defects of vision arise.
There are two types of defects of vision :
(1)Near-sightedness (Myopia)
(2)Far-sightedness
NEAR-SIGHTEDNESS
If the lens of the eye does not become sufficiently thin to focus a distant object, the rays coming from the object after being refracted are focussed before reaching the retina.
As the lens remains thick, it can focus nearby objects perfectly on the retina. Therefore the defect is called nearsightedness.
This defect arises due to excessive convergence of the light rays.
A concave lens of appropriate focal length corrects this defect.
FAR-SIGHTEDNESS
If the lens of the eye does not become sufficiently thick to focus a nearby object, the rays coming from the object, after being refracted are focussed behind the retina.
As the lens remains thin, it can focus distant objects perfectly on the retina. Therefore the defect is called far-sightedness.
This defect arises due to less convergence of the light rays.
A convex lens of appropriate focal length corrects this defect.
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