Question

A person can see clearly only up to 3metres .precribe a lens for spectacles so that he can see up to 12metres ?

Answer 1

Answer here

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given :-

v=>-3m

u=>-12m

f=>?

_______________

using

1/v -1/u =>1/f

1/-3-1/-12=>1/f

=>1/-3+1/12=>1/f

f=>-4m

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a concave lens of focal length 4m should be used

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Answer 2

According to the problem the the man can see up to 3 M

therefore V = - 3 M

now to see the objects present up to 12 M

therefore U = - 12 M

We need to find the focal length ,f

1/f = 1/V - 1/U

   = 1/(-3) - 1/(-12)

   = - (4-1)/12

  = - (1/4)

Now we need to calculate the power

P = 1/f

   = 1 / - (1/4)

   = - 4 D

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