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Wave Optics
Question
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The magnifying power of a telescope is 9. When it is adjusted for parallel rays the distance between the objective and eyepiece is 20 cm. The focal length of lenses are

  • 10 cm, 10 cm

  • 15 cm, 5 cm

  • 18 cm, 2cm

  • 11 cm, 9 cm

Answer

C.

18 cm, 2cm

Given M = fo/fe =9 and fo+fe = 20
fo = 9fe
So, 9fe+fe = 20
fe = 2cm
fo = 9 x2
fo =18

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