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प्रश्न
The far point of a myopic person is 80 cm in front of the eye. What is the nature and power of the lens required to correct the problem?
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उत्तर
The person is suffering from an eye defect called myopia. In this defect, the image is formed in front of the retina. Hence, a concave lens is used to correct this defect of vision.
Object distance, u = infinity = ∞
Image distance, v = – 80 cm
Focal length = f
According to the lens formula,
`1/"v" - 1/"u" = 1/"f"`
`1/(-80) - 1/(-∞) = 1/"f"`
`1/(-80) = 1/"f"`
f = −80 cm = −0.80 m ...(concave lens)
∴ power of modified lens,
P = `1/"f"`
= `1/(-0.80 "m")`
= −1.25 D
A concave lens of power −1.25 D is required by the person to correct his defect.
संबंधित प्रश्न
A student suffering from myopia is not able to see distinctly the objects placed beyond 5 m. List two possible reasons due to which this defect of vision may have arisen. With the help of ray diagrams, explain
(i) why the student is unable to see distinctly the objects placed beyond 5 m from his eyes.
(ii) the type of the corrective lens used to restore proper vision and how this defect is corrected by the use of this lens.
(b) If, in this case, the numerical value of the focal length of the corrective lens is 5 m, find the power of the lens as per the new Cartesian sign convention
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|
Column I
|
Column II
|
Column III
|
|
(i) Myopia
|
Old age problem
|
Bifocal lens
|
|
(ii) Presbyopia
|
Nearsightedness
|
Concave lens.
|
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- Draw the diagram to show the correction of this defect.
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- Identify the defect of vision shown.
- List its two causes.
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