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The least distance of vision of a longsighted person is 60 ~cm . By using a spectacle lens, this distance is reduced to 12 ~cm . The power of the lens is

Options

  1. A+5.0 D
  2. B+(20 / 3) D
  3. C-(10 / 3) D
  4. D+2.0 D

Correct answer

B. +(20 / 3) D

Step-by-step solution

Using, 1 f = 1 v - 1 u Here, v=-60 ~cm , u=-12 ~cm . gathered 1 f = 1 (-60) - 1 (-12) = -1 60 + 1 12 = 1 15 1 f = 1 15 f=15 ~cm gathered Power, P= 100 15 = 20 3 D

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