Foundation
Physics Foundation
Human Eye And Colorful World
Question

(a) What is short-sightedness? State the two causes of short-sightedness (or myopia). With the help of ray diagrams, show :
(1) the eye-defect short-sightedness.
(2) correction of short-sightedness by using a lens.
(b) A person having short-sightedness cannot see objects clearly beyond a distance of 1.5 m. What would be the nature and power of the corrective lens used  to restore proper vision?

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Solution
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(a) Short Sightedness is that defect of vision due to which a person can see nearby objects clearly but cannot see distant objects clearly.
Causes:
(1) Excessive curvature of the eye lens
(2) Elongation of the eyeball
Ray diagram for the below two are given above.
(i) eye-defect short sightedness

(ii) correction of short-sightedness by using a lens.
(b) Concave lens should be used to restore proper vision.
Calculation of power :
Object distance, u = –∞
Image distance, v = –1.5 m
1v-1u=1f
1-1.5-1-=1f
f = –1.5 m
P=1f=1-1.5=-0.67D
In a myopic eye, image of distant object is formed in fornt of the retina (and not on the retina)
The far point (F) of a myopic eye is less than infinity
(ii) Correction of short-sightedness by using a lens
Correction of myopia, The concave lens placed in front of the eye forms a virtual image of distant object at far point (F) of the myopic eye.
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