When the tube length of an astronomical telescope is increased,its magnifying power will .......

If the length of the tube of an astronomical telescope is $105 \, cm$ and its magnification power for normal adjustment is $20$,then the focal length of its objective lens will be ....... $cm$.

An astronomical refracting telescope is being used by an observer to observe planets in normal adjustment. The focal lengths of the objective and eyepiece used in the construction of the telescope are $20\,m$ and $2\,cm$ respectively. Consider the following statements about the telescope:
$(a)$ The distance between the objective and eyepiece is $20.02\,m$.
$(b)$ The magnification of the telescope is $1000$.
$(c)$ The image of the planet is erect and diminished.
$(d)$ The aperture of the eyepiece is smaller than that of the objective.
The correct statements are:

The diameter of the moon is $3.5 \times 10^{3} \, km$ and its distance from the Earth is $3.8 \times 10^{5} \, km$. It is viewed by a telescope with objective and eyepiece focal lengths of $4 \, m$ and $10 \, cm$ respectively. What is the angular diameter of the image of the moon in degrees?

The objective and eyepiece of an astronomical telescope are double convex lenses with refractive index $1.5$. When the telescope is adjusted to infinity, the separation between the two lenses is $16 \,cm$. If the space between the lenses is now filled with water and the telescope is again adjusted for infinity, then the present separation between the lenses is: (in $\,cm$)

If an astronomical telescope has objective and eye-piece focal lengths of $200 \; cm$ and $4 \; cm$ respectively,then the magnifying power of the telescope for normal vision is: