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प्रश्न
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?
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उत्तर
A person suffering from hypermetropia can correct the defect by wearing spectacles containing convex lenses. In order to find the power of the convex lens required, we have to first calculate its focal length.
Given that the near point of the hypermetropic person is 0.75 m from the eye (the person can see objects kept at the normal near point of 0.25 m in front of the eye if the images of the objects are formed at the person's own near point of 0.75 m from the eye).u=-0.25m(the distance of the object at the normal near point)v=-0.75m (the near point of the defective eye in front of the lens)
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/-0.75-1/-0.25`
`1/f=((-1+3)/0.75)`
∴f =2.67 m
Now that we know the focal length of the convex lens, its power can be calculated.
Poer `P=1/(f "in meters")`
Hence, the power of the convex lens required to rectify the defect is +0.37 D.
This eye is long-sighted.
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संबंधित प्रश्न
Name the defect of vision in a person:
whose far point is less than infinity
What is the scientific name of
long-sightedness?
What is short-sightedness? State the two causes of short-sightedness (or myopia). With the help of ray diagrams, show:
(i) the eye-defect short-sightedness.
(ii) correction of short-sightedness by using a lens.
The defect of vision which cannot be corrected by using spectacles is:
(a) myopia
(b) presbyopia
(c) cataract
(d) hypermetropia
Though a woman can see the distant object clearly, she cannot see the nearby objects clearly. She is suffering from the defect of vision called:
(a) long-sight
(b) short-sight
(c) hind-sight
(d) mid-sight
The near point of a long-sighted person is 50 cm from the eye.
(a) Can she see clearly an object at:
(i) a distance of 20 cm?
(ii) at infinity?
Explain the terms ‘adaptation’ and ‘accommodation’ with reference to the eye.
Enumerate the common defects of vision, their causes and the possible methods of correcting them.
State the main functions of the following:
Tears
List two causes of presbyopia. Draw labelled diagram of a lens used for the correction of this defect of vision.
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.
Given below is a diagram depicting a defect of the human eye? Study the same and answer the question that follow:
Give two possible reasons for this defect of the eye in human beings.
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?
Give Reason:
When you enter into a dark room from bright because of sunlight, you can not see things for a few seconds.
Explain the Term: Hypermetropia
Nearsightedness : concave lens : : farsightedness : _______
Write scientific reason.
Nearsightedness, this defect can be corrected by using spectacles with concave lens.
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.
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?
