Optics Question 392

Question: The magnifying power of a telescope is 9. When it is adjusted for parallel rays, the distance between the objective and the eye-piece is found to be 20 cm. The focal length of the two lenses are

[MP PMT 1986]

Options:

A) 18 cm, 2 cm

B) 11 cm, 9 cm

C) 10 cm, 10 cm

D) 15 cm, 5 cm

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Answer:

Correct Answer: A

Solution:

In this case $ |m|\ =\frac{f _{o}}{f _{e}}=9 $ …. (i)

and length of telescope $ =f _{o}+f _{e}=20 $ …. (ii)

Solving (i) and (ii), we get fe = 2 cm, $ f _{o}=18cm. $



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