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प्रश्न
An eye has a far point of 2 m. What type of lens in spectacles would be needed to increase the far point to infinity? Also calculate the power of lens required. Is this eye long-sighted or short-sighted?
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उत्तर
The far point of an eye can be increased by wearing spectacles with concave lens. In order to find the power of the concave lens required, we have to first calculate its focal length.
Given that the far point of the short-sighted person is 2 m in front of the eye (the person can see objects kept at infinity if the images of the objects are formed at the person's own far point of 2 m from the eye).
`u=oo("the distance of the object")`
`v=-2m("the far point of the defective using the formula ")1/f=1/v-1/u`
f=? (focal length)
The focal length can be calculated using the formula `1/f=1/v-1/u`
Substituting the values in the formula, we get `1/f=1/-2-1/oo=(-1)/2 ("Sinc e, 1/oo=0")`
∴ f=-2 m
Now that we know the focal length of the concave lens required, its power can be calculated.
Power `P=1/f` (for f in meters)
∴` P=1/-2=-0.5 D`
Hence, the power of the concave lens required to rectify the defect is -0.5 D.
This eye is short-sighted.
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संबंधित प्रश्न
Explain with the help of labelled ray diagram, the defect of vision called myopia and how it is corrected by a lens.
A person having short-sight cannot see objects clearly beyond a distance of 1.5 m. What would be the nature and power of the corrective lens to restore proper vision?
What is long-sightedness? State the two causes of long-sightedness (or hypermetropia). With the help of ray diagrams, show:
(i) the eye-defect long-sightedness.
(ii) correction of long-sightedness by using a lens.
An eye has a near point distance of 0.75 m. What sort of lens in spectacles would be needed to reduce the near point distance to 0.25 m? Also calculate the power of lens required. Is this eye long-sighted or short-sighted?
The defect of vision which cannot be corrected by using spectacles is:
(a) myopia
(b) presbyopia
(c) cataract
(d) hypermetropia
Observer the following diagram and answer the questions.
a) Which eye defect is shown in this diagram?
b) What are the possible reasons for this eye defect?
c) How this defect is corrected, write it in brief?
List two causes of presbyopia. Draw labelled diagram of a lens used for the correction of this defect of vision.
When do we consider a student sitting in the class to be myopic? List two causes of this defect. Explain using a ray diagram how this defect of eye can be corrected.
Distinguish between the following pair of words:
Myopia and hypermetropia
Write whether the following is true or false:
A convex lens is used for correcting myopia.
In what two whys is a yellow spot different from the blind spot?
Name the common defects of the eye.
Give Reason:
Older people require glasses to read and write.
Choose the Odd One Out:
Due to elongation of _______ and increase in curvature of the eye lens, a person cannot see distant objects clearly.
Nearsightedness : concave lens : : farsightedness : _______
Which of the following statement is correct?
A student sitting at the back of the classroom cannot read clearly the letters written on the blackboard. What advice will a doctor give to her? Draw ray diagram for the correction of this defect.
Name the following:
Two kinds of accomodations.
