The magnifying power of a telescope with tube length $60 \; cm$ is $5$. What is the focal length of its eyepiece in $cm$?

The final image formed by an astronomical telescope is

There are four convex lenses $L_{1}, L_{2}, L_{3}$ and $L_{4}$ with focal lengths $2 \ cm, 4 \ cm, 6 \ cm$ and $8 \ cm$ respectively. Two of these lenses form a telescope of length $10 \ cm$ and magnifying power $4$. The objective and eye lenses are respectively:

An astronomical telescope has objective and eye-piece lenses of powers $0.5\, D$ and $20\, D$ respectively. Its magnifying power will be:

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

Which are two main points important for an astronomical telescope?