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?

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Chapter 10

NCERT

Class 10

Science

Solutions

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

Question:

A person needs a lens of power –5.5 diopters 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) for far sight :

P = – 5.5 D

Power of lens = 1/f (in m)

∴ f = 1/P

f = 1/(-5.5)

f = -10/15

f = – 0.182 = – 0.18 m

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(ii) for near sight

P = + 1.5

∴ P = 1/f (in m)

∴ f = 1/P

f = 1/1.5

f = 10/15

f = 2/3

f = 0.66 cm = 0.67 cm

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