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NEET 2026
Exam
NEET
Year
2026
Subject
Physics
Chapter
Chapter

NEET 2026 · Physics · Chapter

An astronomical telescope is set in normal adjustment for viewing a...

An astronomical telescope is set in normal adjustment for viewing a distant object. The magnifying power of the telescope is 1414 and the distance between the objective lens and the eyepiece is 105 cm105\text{ cm}. The focal lengths of the objective and the eyepiece are respectively:

Options

  1. A100 cm100\text{ cm} and 5 cm5\text{ cm}
  2. B7 cm7\text{ cm} and 98 cm98\text{ cm}
  3. C98 cm98\text{ cm} and 7 cm7\text{ cm}
  4. D91 cm91\text{ cm} and 14 cm14\text{ cm}

Correct answer

C. 98 cm98\text{ cm} and 7 cm7\text{ cm}

Step-by-step solution

For an astronomical telescope in normal adjustment, the magnifying power is given by m=fofem = \dfrac{f_o}{f_e} and the length of the telescope tube is L=fo+feL = f_o + f_e.

Given m=14m = 14 and L=105 cmL = 105\text{ cm}.

fofe=14⇒fo=14fe\dfrac{f_o}{f_e} = 14 \Rightarrow f_o = 14 f_e

Substituting this into the length equation:

14fe+fe=10514 f_e + f_e = 105

15fe=105⇒fe=7 cm15 f_e = 105 \Rightarrow f_e = 7\text{ cm}

fo=14×7=98 cmf_o = 14 \times 7 = 98\text{ cm}

The focal lengths of the objective and the eyepiece are 98 cm98\text{ cm} and 7 cm7\text{ cm} respectively.

Answer: 98 cm98\text{ cm} and 7 cm7\text{ cm}
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