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Question

The magnifying power of a telescope with a tube length of 60cm is 5. What is the focal length of its eyepiece?


A

10cm

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B

20cm

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C

30cm

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D

40cm

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Solution

The correct option is A

10cm


Step 1. Given data:

The magnifying power of the telescope m=5.

The length of the tube=60cm.

Step 2. Formula used:

m=fefo where the focal length of the eyepiece lens is fe, and the focal length of the objective lens is fo

The total length of the telescope=fe+fo

Step 3. Calculating the focal length of the eyepiece,

m=fofe=5fo=5fe

And,

fe+fo=60fe+5fe=606fe=60fe=606=10

Thus the focal length of the eyepiece is10cm

Hence, option A is the correct answer.


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