Question

a person needs a lens of power -5.5 for correcting his distant vision for correcting is near vision he is the length of power plus 1.5 what is the focal length of the lens required for correcting first distant vision and second year vision​

Answer 1

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Answer 2

Given that, a person needs a lens of power -5.5 for correcting his distant vision.

And for correcting his near vision he is the length of power +1.5.

We have to find the focal length of the lens required for correcting first distant vision and second near vision.

Now,

For distant vision:

P = -5.5 and focal length is in meter.

P = 1/f

-5.5 = 1/f

f = 100/(-5.5) = -18.18 cm

Therefore, a person needs a concave lens for correcting his distant vision. (having, f = 18.18 cm)

For near vision:

P = +1.5

P = 1/f

f = 100/1.5 = +66.67 cm

Therefore, a person needs a convex lens for correcting his near vision. (having f = 66.67 cm)

Power of lens (P) is defined as the reciprocal of its focal length (f).

P = 1/f

For convex lens, Power of lens is positive and for the concave lens, it is negative.

S.I. unit of power of lens is Dioptre (D).