First slide

The optical instruments

Question

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

Easy

A

20cm
B

10cm
C

30cm
D

40cm

Solution

$\begin{array}{l} M a g n i f y i n g p o w e r = m = \frac{f_{o}}{f_{e}} \\ \Rightarrow 5 = \frac{f_{o}}{f_{e}} \\ \Rightarrow f_{o} = 5 f_{e} \\ L e n g t h o f t u b e = f_{o} + f_{e} = 60 c m \\ \Rightarrow 6 f_{e} = 60 \\ \Rightarrow f_{e} = 10 c m \end{array}$

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