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[1]
A person needs a lens of power $- 5.5 D$ to correct his distant vision. What is the focal length of the lens required?

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- 18 cm

+ 18 cm

- 9 cm

+ 9 cm

answer is A.

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Detailed Solution

A person needs a lens of power

- 5.5 D

for correcting his distant vision. The focal length of the lens required for correcting distant vision is

- 18 cm

.
We know that focal length (in metres) is the inverse of the power of the lens. It is given that the power of the lens is

- 5.5

Dioptre

for correcting distant vision. The lens's focal length for distant vision is the inverse of power.

Focal length (f) in metres = \frac{1}{Power}

\Rightarrow f = \frac{1}{- 5.5 D}

\Rightarrow f = - 0.18 m

\Rightarrow f = - 18 cm

(as

1 m = 100 cm

)

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