If the tube length of astronomical telescope is 105 cm long and magnifying power

Question

If the tube length of astronomical telescope is 105 cm long and magnifying power is 20 for normal setting. Calculate the focal length of objective.

Options:

A . 100 cm

B . 10 cm

C . 20 cm

D . 25 cm

Answer: Option A
:
A

m = \frac{f_{0}}{f_{e}}; 20 = \frac{f_{0}}{f_{e}} f_{e} = \frac{f_{0}}{20} L = f_{0} + f_{e} 105 = f_{0} + f_{e} 105 = f_{0} + \frac{f_{0}}{20} 105 = (\frac{20 + 1}{20}) f_{0} f_{0} = \frac{105 \times 20}{21} f_{0} = 100 c m

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