If the tube length of an astronomical telescope is $105 \, cm$ and the magnifying power is $20$ for normal setting,calculate the focal length of the objective in $cm$.

$A$ small object is placed at a distance of $4 \,m$ from the objective of a telescope of focal length $2 \,m$. The focal length of the eyepiece is $0.2 \,m$. The final image of the object

An astronomical telescope has an eyepiece of focal length $5 \ cm$. The angular magnification in normal adjustment is $10$. When the final image is formed at the least distance of distinct vision $(25 \ cm)$ from the eyepiece,then the angular magnification will be:

The number of lenses in a terrestrial telescope is

Four lenses of focal length $+ 15\, cm, + 20\, cm, + 150\, cm$ and $+ 250\, cm$ are available for making an astronomical telescope. To produce the largest magnification,the focal length of the eye-piece should be.....$cm$

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