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  • 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?

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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For the distant vision:

The power of the lens, given P = -5.5 D

Power(in D) = frac1f(m)

Hence the focal length will be:

f= frac100-5.5 = -18.2 cm

Therefore, the focal length of the lens required for correcting distant vision will be:-18.2 cm (Here, the negative sign of focal length tells us that it is a concave lens).

 

For near vision:

The power of the lens, given P = +1.5 D

Power(in D) = frac1f(m)

Hence the focal length will be:

f= frac100+1.5 = 66.66 cm

Therefore, the focal length of the lens required for correcting near vision will be:+66.7 cm (Here, the positive sign of focal length tells us that it is a convex lens).

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Deependra Verma

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