Question 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:
For distant vision = −0.181 m, for near vision = 0.667 m
The power P of a lens of focal length f is given by the relation
(i) Power of the lens used for correcting distant vision = −5.5 D
Focal length of the required lens, f = 1/P
The focal length of the lens for correcting distant vision is −0.181 m.
(ii) Power of the lens used for correcting near vision = +1.5 D
Focal length of the required lens, f = 1/P
The focal length of the lens for correcting near vision is 0.667 m.
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