An astronomical telescope and a Galilean telescope use identical objective lenses. They have the same magnification, when both are in normal adjustment. The eyepiece of the astronomical telescope has a focal length f. Select the correct statement(s).

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The Galilean telescope has shorter tube length.

The Galilean telescope has longer tube length.

The tube lengths of the two telescopes differ by f.

The tube lengths of the two telescopes differ by 2f.

answer is B, C.

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Detailed Solution

The tube length of an astronomical telescope, in normal adjustment, is $(f_{0} + f_{e})$ and that of Galilean telescope, in normal adjustment is $(f_{0} - f_{e})$ where $f_{0}$ and $f_{e}$ are focal lengths of objective and eye piece respectively. In this case, $f_{e} = f$ ,so difference in tube lengths is
$l_{1} - l_{2} = (f_{0} + f_{e}) - (f_{0} - f_{e}) = 2 f_{e} = 2 f$