# The magnifying power of a telescope with tube length 60 cm is 5. What is the focal length of its eyepiece: A. 30 cm B. 10 cm C. 40 cm D. 20 cm

## Question

The magnifying power of a telescope with tube length 60 cm is 5. What is the focal length of its eyepiece: A. 30 cm B. 10 cm C. 40 cm D. 20 cm

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## Answer

The correct answer is **B**. Here is the detailed solution:

To determine the focal length of the eyepiece, we use the magnifying power formula of a telescope:

$$ m = \frac{f_o}{f_e} $$

Given, the magnifying power ( m ) is **5**:

$$ 5 = \frac{f_o}{f_e} \implies f_o = 5 f_e $$

We are also given that the **tube length** is **60 cm**. According to the formula:

$$ f_o + f_e = 60 $$

Substituting ( f_o ) from the earlier equation:

$$ 5 f_e + f_e = 60 \implies 6 f_e = 60 \implies f_e = 10 , \text{cm} $$

Thus, the focal length of the eyepiece is **10 cm**.

### Final Answer: B

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