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प्रश्न
A student suffering from myopia is not able to see distinctly the objects placed beyond 5 m. List two possible reasons due to which this defect of vision may have arisen. With the help of ray diagrams, explain
(i) why the student is unable to see distinctly the objects placed beyond 5 m from his eyes.
(ii) the type of the corrective lens used to restore proper vision and how this defect is corrected by the use of this lens.
(b) If, in this case, the numerical value of the focal length of the corrective lens is 5 m, find the power of the lens as per the new Cartesian sign convention
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उत्तर
Two possible reasons due to which this defect of vision may have arisen are
(a) increase in curvature of the lens
(b) increase in length of the eyeball
(i) A myopic eye has its far point nearer than infinity. It forms the image of a distant object in front of its retina as shown in the figure given below. In the given case student's far point is 5 m. So, image of the object placed beyond 5 m from his eyes is formed in front of the retina and hence appears blurred. That is why the student is unable to see distinctly the objects placed beyond 5 m from his eyes.
(ii) Since a concave lens has an ability to diverge incoming rays, it is used to correct this defect of vision. The image is allowed to form at the retina by using a concave lens of suitable power as shown in the given figure.
(b) Power of the required corrective lens (P) =`1/(f("in m"))`
`P =− 1/5 `
`P= −0.2 D`
Hence, the power of corrective lens is -0.2 D.
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संबंधित प्रश्न
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
(ii) for its correction.
The far point of a myopic person is 80 cm in front of the eye. What is the nature and power of the lens required to correct the problem?
Which defect of vision can be rectified:
by using a concave lens?
Describe the mechanism of focusing the image of a distant object in your eye when you raise your head after reading a book.
You do not enjoy watching a movie from a very short distance from the screen in a cinema hall. Why?
List two causes of presbyopia. Draw labelled diagram of a lens used for the correction of this defect of vision.
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.
Write scientific reason.
Nearsightedness, this defect can be corrected by using spectacles with concave lens.
Match the following:
| Column - I | Column - II |
| 1. Retina | a. Path way of light |
| 2. Pupil | b. Far point comes closer |
| 3. Ciliary muscles | c. near point moves away |
| 4. Myopia | d. Screen of the eye |
| 5. Hypermetropia | e. Power of accommodation |
