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Myopia (short sightness): A short-sighted eye can see only closer bodies. Distant bodies are not seen clearly.
(i) In this defect picture is created before the retina and Far point comes closer.
(ii) In this defect focal length or radii of curvature of lens reduced or power of lens increases or distance between eye lens and retina increases.
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(iii) This defect may be replaced by using a concave lens of suitable focal length.
(iv) If defected far position is at a distance d from eye then
Focal length of used lens f = -d = -(defected far point)
(v) A person can see up to distance → x, have to look distance → y(y>x)
so f=xy/x-y or power of the lens p = x-y/xy
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Hypermetropia (long sightness): A long-sighted eye can see distance objects clearly but nearer object are not clearly visible.
(i) Image created behind the retina and closer point moves away
(ii) In this defect focal length or radii of curvature of lens increases or power of lens decreases or distance between eye lens and retina decreases.
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(iii) This defect may be replaced by using a convex lens.
(iv) If a person cannot look before distance d but needs to look the object placed at distance D from eye so f= dD/d-D and power P = d-D/dD of the lens
Presbyopia: In this defect both near and far objects are not clearly visible. It is an old age disease and it is due to the losing power of accommodation. It may be replaced by using bifocal lens.
Astigmatism: In this defect eye cannot see horizontal and vertical lines clearly, simultaneously. It is due to imperfect spherical behavior of eye lens. This defect may be replaced by using cylindrical lens (Torric lenses).
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