The focal length of the objective and eyepiece of a telescope are respectively $60 \, cm$ and $10 \, cm$. The magnitude of the magnifying power when the image is formed at infinity is:

What is the diameter of the objective used in astronomical telescopes in present use (in $cm$)?

The length of an astronomical telescope for normal vision (relaxed eye) ($f_o$ = focal length of objective lens and $f_e$ = focal length of eye lens) is

In an astronomical telescope in normal adjustment,a straight black line of length $L$ is drawn on the inside part of the objective lens. The eyepiece forms a real image of this line. The length of this image is $I$. The magnification of the telescope is

An astronomical refracting telescope has an objective of focal length $20 \, m$ and an eyepiece of focal length $2 \, cm$. Then:

An astronomical telescope has a magnifying power $10$. The focal length of the eyepiece is $20 \, cm$. The focal length of the objective is.....$cm$.