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प्रश्न
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.
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उत्तर
Long-sightedness, or hypermetropia, is a defect of vision because of which a person cannot see nearby objects clearly but has normal distant vision. Hypermetropia occurs because
1. the eye lens has low converging power, which causes the formation of the images of objects behind the retina, or
2. the eye ball is too short, which causes the formation of the images of objects behind the retina
The ray diagram shows the hypermetropic eye and how the defect is corrected by using a convex lens, which converges the light rays from objects and forms clear images on the retina.
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संबंधित प्रश्न
In a Std. X class out of 40 students 10 students use spectacles, 2 students have positive power and 8 students have negative power of lenses in their spectacles.
Answer the following questions:
(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.
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.
Explain two possible reasons of myopia. How can it be corrected? Explain with a suitable diagram.
The kind of lens required to correct Myopia
the biological/technical terms for the lens of eye losing flexibility resulting in a kind of long-sightedness in middle aged people.
Name one defect of vision (or eye) which cannot be corrected by any type of spectacle lenses.
Name the defect of vision which can be corrected by a converging lens. Show clearly by a ray diagram how the lens corrects the defect.
The near-point of a person suffering from hypermetropia is at 50 cm from his eye. What is the nature and power of the lens needed to correct this defect? (Assume that the near-point of the normal eye is 25 cm).
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.
State the main functions of the following:
Tears
Differentiate between:
Myopia and Hypermetropia.
Give Technical Term:
The path which responsible for protecting the eye from sweat.
In Myopia the human eye _______.
Write scientific reason.
Nearsightedness, this defect can be corrected by using spectacles with concave lens.
Assertion: Myopia is the defect of vision in which a person cannot see distant objects clearly.
Reason: This due to eye-ball being too short.
Myopia may arise due to ____________.
State reasons for Myopia. With the help of ray diagrams, show the:
- image formation by a myopic eye, and
- correction of myopia using an appropriate lens.
