An object is clearly seen through an astronomical telescope of length $50 \ cm$. The focal lengths of its objective and eyepiece,respectively,can be

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 ....... $cm$.

$(a)$ $A$ small telescope has an objective lens of focal length $140\;cm$ and an eyepiece of focal length $5.0\;cm$. What is the separation between the objective lens and the eyepiece?
$(b)$ If this telescope is used to view a $100\;m$ tall tower $3\;km$ away,what is the height of the image of the tower formed by the objective lens?
$(c)$ What is the height of the final image of the tower if it is formed at $25\;cm$?

What is the angular magnification of a telescope if the focal lengths of the objective and eye lenses are $10 \ cm$ and $10 \ mm$ respectively,and the tube length is $11 \ cm$?

$A$ reflecting telescope utilizes

Two convex lenses of focal lengths $0.3 \, m$ and $0.05 \, m$ are used to make a telescope. The distance kept between the two is .... $m$