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 - Science

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Numerical

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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Solution

(i) In order to correct distant vision, a person wears spectacles with concave lenses.

Given that the power of the concave lens is required to correct the distant vision of the person, P = −5.5 D.

The focal length of the concave lens required for correcting the defect is given by f = `1/p`

∴ f = `1/-5.5`

= −0.18 m

= −18 cm

Hence, the focal length of the concave lens required to correct the person's distant vision is −18 cm.

(ii) In order to correct near vision, a person wears spectacles with convex lenses.

Given the power of the convex lens, P = +1.5 D.

The focal length of the convex lens required for correcting this defect is given by f = `1/p` 

f = `1/(1.5)`

= 0.67 m

= 67 cm

Hence, the focal length of the convex lens required to correct his near vision is 67 cm.

Concept: Eye Defect and Its Correction: Presbyopia
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Chapter 6: The Human Eyes And The Colorful World - Exercise 2 [Page 280]

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Lakhmir Singh Class 10 Physics (Science)
Chapter 6 The Human Eyes And The Colorful World
Exercise 2 | Q 29 | Page 280
NCERT Science Class 10
Chapter 11 The Human Eye and the Colourful World
Exercises | Q 5 | Page 198
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