Question

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

Answer 1

Correct option is C. $$10cm$$
The magnifying power of a telescope is given by

$$m = \dfrac{f_0}{f_e}, \, 5 = \dfrac{f_0}{f_e}$$

$$f_0 + f_e = 60$$ [The total length of the telescope is $$60cm$$]

And $$f_0 = 5f_e$$

$$\therefore f_0 + f_e = 5f_0, \, 6f_e = 60, f_e = 10cm$$

$$f_0 = 5f_e = 50cm$$

$$\therefore$$ The focal length of the eyepiece is $$f_e = 10cm$$