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CBSE Class 10 Science 2017 Outside Delhi Term II Set 1 Paper
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Question : 24 of 36
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(a) A student suffering from myopia is not able to see distinctly the objects placed beyond . 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 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 , find the power of the lens as per the new Cartesian sign convention.
(i) Why the student is unable to see distinctly the objects placed beyond 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 , find the power of the lens as per the new Cartesian sign convention.
Solution:
(a) Two possible reasons due to which this defect of vision may have arisen are :
1. Increase in curvature of the lens.
2. Elongation 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 the retina as shown below :
(ii) Since a concave lens has an ability to diverge the incoming rays. Therefore, it is used to correct this defect of vision. The image is formed at the retina by the use of a concave lens of suitable power as shown.
1. Increase in curvature of the lens.
2. Elongation 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 the retina as shown below :
In the given case, student's far point is 5 . So, image of the object placed beyond from his eyes is formed in front of the retina and object appears blurred. That is why the student is unable to see distinctly the objects placed beyond from his eye.
(ii) Since a concave lens has an ability to diverge the incoming rays. Therefore, it is used to correct this defect of vision. The image is formed at the retina by the use of a concave lens of suitable power as shown.
(b)
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