The focal length of eye lens and object lens of a telescope is $4$ mm and $4$ cm respectively. If final image of a far object is at $\infty$ . Which of the following is correct for the magnifying power and length of the tube?

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4

44

10

4

4

44

, 10 cm

10

, 44 cm

answer is B.

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Detailed Solution

The focal length of eye lens and object lens of a telescope is

4

mm and

4

cm respectively. If final image of a far object is at

\infty

. Then the magnifying power and length of the tube is 10,

4.4

cm respectively.
The focal lengths of the telescope's eye lens and object lens are

4

mm = 0

4

cm and

4

cm, respectively. So, magnification M=

\frac{f_{o}}{f_{e}}

\frac{4}{0.4}

= 10. The focal length of the eyepiece is the distance from the centre of the eyepiece lens to the point at which light passing through the lens is brought into focus. The focal length of the objective is the distance from the centre of the objective lens (or mirror) to the point at which the incoming light is brought into focus. The length of the tube is given as the sum of the focal lengths of the eye lens and the object lens. So, length of tube is