In normal adjustment, for a refracting telescope, the distance...

In normal adjustment, for a refracting telescope, the distance between objective and eye piece is 30 cm. The focal length of the objective, when the angular magnification of the telescope is 2 , will be:

Options

A20 cm
B30 cm
C10 cm
D15 cm

Correct answer

A. 20 cm

Step-by-step solution

\because \mathrm{f}_0+\mathrm{f}_{\mathrm{e}}=30

And magnification,

\mathrm{m}=\frac{\mathrm{f}_0}{\mathrm{f}_e}2=\frac{\mathrm{f}_0}{\mathrm{f}_{\mathrm{e}}} \Rightarrow \mathrm{f}_0=2 \mathrm{f}_{\mathrm{e}} \Rightarrow \mathrm{f}_0+\frac{\mathrm{f}_0}{2}=30 \therefore \mathrm{f}_0=20cm

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