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प्रश्न
A person suffering from the eye-defect myopia (short-sightedness) can see clearly only up to a distance of 2 metres. What is the nature and power of lens required to rectify this defect?
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उत्तर
A person suffering from myopia can correct the defect by wearing spectacles with concave lenses. 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 myopic person is 2 m from the eye (the person can see an object kept at infinity if the image of the object is formed at the far point of 2 m from the eye).
u = ∞ (distance of the object)
v = -2 m (far point in front of the lens)
f = ? (focal length)
The focal length can be calculated using the lens formula `1/v-1/u=1/f`
Substituting the values in the formula, we get `1/-2-1/oo=1/f`
∴ f = -2 m
Now that we know the focal length of the concave lens, the power can be calculated.
Power P=`1/f("inmeters")`
p=`1/-2=-0.5` diptres
Therefore, the power of the concave lens required to rectify the defect is -0.5 dioptres.
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संबंधित प्रश्न
Do you know that the corneal-impairment can be cured by replacing the defective cornea with the cornea of the donated eye? How and why should we organise groups to motivate the community members to donate their eyes after death?
Millions of people in the developing countries of the world are suffering from corneal blindness. These persons can be cured by replacing the defective cornea with the cornea of a donated eye. A charitable society of your city has organised a campaign in your neighbourhood in order to create awareness about this fact. If you are asked to participate in this mission, how would you contribute in this noble cause?
(i) State the objective of organising such campaigns.
(ii) List two arguments which you would give to motivate the people to donate their eyes after death.
(iii) List two values which are developed in the persons who actively participate and contribute in such programmes.
What is the far point and near point of the human eye with normal vision?
What is the scientific name of
short-sightedness
Name one defect of vision (or eye) which cannot be corrected by any type of spectacle lenses.
Name the body part with which the terms myopia and hypermetropia are connected.
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?
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(i) the eye-defect long-sightedness.
(ii) correction of long-sightedness by using a lens.
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(a) cataract
(b) hypermetropia
(c) myopia
(d) presbyopia
Name the following:
The eye defect caused due to shortening of the eye ball from front to back.
Differentiate between members of the following pair with reference to what is asked in the bracket.
Rods and cones (sensitivity).
Draw a ray diagram to show how Hypermetropia is defect can be corrected using a lens.
A student has difficulty reading the blackboard while sitting in the last row. What could be the defect the child is suffering from? How can it be corrected?
Rewrite the following table so as to match second and third column with first column.
|
Column I
|
Column II
|
Column III
|
|
(i) Myopia
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Old age problem
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Bifocal lens
|
|
(ii) Presbyopia
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Nearsightedness
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Concave lens.
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(a) Name the defect of vision the person is suffering from and list its two possible causes.
(b) Draw a ray diagram to show the defect in the above case.
(c) Mention the type of lens used by him for the correction of the defect and calculate its power. Assume that the near point for the normal eye is 25 cm.
(d) Draw a labeled diagram for the correction of the defect in the above case.
Differentiate between:
Short-sightedness and Long-sightedness.
Study the following diagram carefully and then answer the questions that follow. The diagram is depicting a defect of the human eye :
(i) Identify the defect shown in the diagram.
(ii) Give two possible reasons for the above defect.
Assertion: Concave mirrors are used as reflectors in torches, vehicle head lights and in search lights.
Reason: When an object is placed beyond the center of curvature of a concave mirror, the image formed is real and inverted.
Which of the following statement is correct?
When do we consider a person to be myopic or hypermetropic? Explain using diagrams how the defects associated with myopic and hypermetropic eye can be corrected?
