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?
NCERT Class X
Science - Science
Chapter _The Human Eye and the Colourful World
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for distant vision 0.18 m focal length of concave lens
for near vision 0.67 m focal length of convex lens
p=1/f
for near vision 0.67 m focal length of convex lens
p=1/f
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Answer:
The power P of a lens of focal length f is given by the relationP= 1/f
(i) Power of the lens used for correcting distant vision = - 5.5 D
Focal length of the required lens, f= 1/Pf= 1/-5.5 = -0.181 m
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
f= 1/1.5 = +0.667 m
The focal length of the lens for correcting near vision is 0.667 m.
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