Q. 5 :- A person needs a lens of power –5.5 dioptres for correcting his distant vision. For correcting his near vision he needs a lens of power +1.5 dioptre. What is the focal length of the lens required for correcting (i) distant vision, and (ii) near vision?
Answer:-
(i) In case of distant vision
1 | ||||
P | = |
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f | ||||
1 | ||||
f | = |
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||
P | ||||
1 | ||||
f | = − |
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5.5 | ||||
10 | ||||
f | = − |
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||
55 | ||||
2 | ||||
f | = − |
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11 | ||||
2 | ||||
f | = − |
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11 | ||||
f | = − | 0.181 | m | |
f | = − | 18.1 | cm |
(ii) In case of near vision.
1 | ||||
P | = |
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||
f | ||||
1 | ||||
f | = |
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||
P | ||||
1 | ||||
f | = − |
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||
1.5 | ||||
10 | ||||
f | = − |
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||
15 | ||||
2 | ||||
f | = |
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||
3 | ||||
f | = | 0.667 | m | |
f | = | 66.7 | cm |