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प्रश्न
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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उत्तर
A person suffering from short-sightedness can correct the defect by wearing spectacles fitted with concave lens. In order to find the power of the concave lens, we have to first calculate its focal length.
Given, the far point of the short-sighted person is 1.5 m from the eye (the person can see the object kept at infinity if the image of the object is formed at the person's own far point of 1.5 m from the eye).
`u=oo("the distance of the object")`
`v=-1.5m("far point of the defective eye in front of thhe lens ")`
`f=? ("focal length")`
The focal length can be calculated using the lens formula `1/f=1/v-1/u`
Substituting the values in the formula, we get
`1/-1.5-1/oo=1/f`
f=-1.5m,
Since,` 1/oo=0`
Now, that we know the focal length of the concave lens, its power can be calculated.
Power `P=1/(f) ("fin metres")`
∴ `P=-1/1.5=-0.667 Dioptres ~~-0.67 "Dioptres"`
Hence, the power of the concave lens required to rectify the defect is -0.67 D.
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संबंधित प्रश्न
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(1) What does the negative power indicate?
(2) What does the positive power indicate?
(3) Generally which type of spectacles do most of the students use?
(4) What defect of eyesight do most of the students suffer from?
(5) Give two possible reasons for the above defect.
A student is unable to see clearly the words written on the blackboard placed at a distance of approximately 4 m from him. Name the defect of vision the boy is suffering from. Explain the method of correcting this defect.
Draw ray diagram for the:-
(i) defect of vision and also
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List three common refractive defects of vision. Suggest the way of correcting these defects.
A student is unable to see clearly the words written on the black board placed at a distance of approximately 3 m from him. Name the defect of vision the boy is suffering from. State the possible causes of this defect and explain the method of correcting it.
What is the other name for
myopia
The defect of vision which cannot be corrected by using spectacles is:
(a) myopia
(b) presbyopia
(c) cataract
(d) hypermetropia
A short-sighted person has a near point of 15 cm and a far point of 40 cm.
(a) Can he see clearly an object at a distance of:
(i) 5 cm?
(ii) 25 cm?
(iii) 50 cm?
(b) To see clearly an object at infinity, what kind of spectacle lenses does he need?
A person can read a book clearly only if he holds it at an arm's length from him. Name the defect of vision:
if the person is a young man
Enumerate the common defects of vision, their causes and the possible methods of correcting them.
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Rewrite the following table so as to match second and third column with first column.
|
Column I
|
Column II
|
Column III
|
|
(i) Myopia
|
Old age problem
|
Bifocal lens
|
|
(ii) Presbyopia
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Nearsightedness
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Concave lens.
|
Solve the following question:
The near point of the eye of a person is 50 cm. Find the nature and power of the corrective lens required by the person to enable him to see clearly the objects placed at 25 cm from the eye?
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Observe the figure and answer the following questions:
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- Draw the diagram to show the correction of this defect.
Observe the following diagram and answer questions following it:
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Name the following:
Two kinds of accomodations.
