The focal length of an objective of a telescope is $3 \ m$ and its diameter is $15 \ cm$. Assuming that for a normal eye,the diameter of the pupil is $3 \ mm$ for its complete use,the focal length of the eyepiece must be: (in $cm$)

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 ....... $cm$.

If both the object and the image are at infinite distances from a refracting telescope,its magnifying power will be equal to:

If the focal length of the objective lens is increased,then:

In a laboratory,four convex lenses ${L_1}, {L_2}, {L_3},$ and ${L_4}$ of focal lengths $2 \ cm, 4 \ cm, 6 \ cm,$ and $8 \ cm$ respectively are available. Two of these lenses form a telescope of length $10 \ cm$ and magnifying power $4.$ The objective and eye lenses are:

For a distant object,the angular magnification of an astronomical telescope is $5$. The distance between the objective and the eyepiece is $36 \, cm$,and the final image is formed at infinity. What are the focal lengths of the objective $(f_o)$ and the eyepiece $(f_e)$?