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Q121 (NDA-II/2024) Science & Technology › Basic Science (Physics, Chemistry, Biology) Official Key

Which one among the following is the correct focal length of a combination of lenses of power 2.5 D and -2·0 D ?

Explanation

The power of a combination of lenses is calculated as the algebraic sum of the individual powers of the lenses. According to the lens formula for combinations:

  • Total Power (P) = P1 + P2
  • Given: P1 = +2.5 D and P2 = -2.0 D
  • P = 2.5 D + (-2.0 D) = +0.5 D

The relationship between the power of a lens and its focal length (f) is expressed by the formula P = 1/f, where the focal length is measured in meters. To find the focal length:

  • f = 1 / P
  • f = 1 / 0.5
  • f = +2.0 m

Since the resultant power is positive, the combination behaves like a converging lens with a focal length of +2.0 m. Thus, Option C is the correct answer.

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