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 | = |
|
||
| f | ||||
| 1 | ||||
| f | = |
|
||
| P | ||||
| 1 | ||||
| f | = − |
|
||
| 5.5 | ||||
| 10 | ||||
| f | = − |
|
||
| 55 | ||||
| 2 | ||||
| f | = − |
|
||
| 11 | ||||
| 2 | ||||
| f | = − |
|
||
| 11 | ||||
| f | = − | 0.181 | m | |
| f | = − | 18.1 | cm | |
(ii) In case of near vision.
| 1 | ||||
| P | = |
|
||
| f | ||||
| 1 | ||||
| f | = |
|
||
| P | ||||
| 1 | ||||
| f | = − |
|
||
| 1.5 | ||||
| 10 | ||||
| f | = − |
|
||
| 15 | ||||
| 2 | ||||
| f | = |
|
||
| 3 | ||||
| f | = | 0.667 | m | |
| f | = | 66.7 | cm | |