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 ....... $cm$.

$A$ telescope has an objective of focal length $50 \text{ cm}$ and an eyepiece of focal length $5 \text{ cm}$. The least distance of distinct vision is $25 \text{ cm}$. The telescope is focused for distinct vision on a scale $200 \text{ cm}$ away. The separation between the objective and the eyepiece is.......$\text{cm}$.

$A$ small telescope has an objective lens of focal length $140 \; cm$ and an eyepiece of focal length $5.0 \; cm$. What is the magnifying power of the telescope for viewing distant objects when
$(a)$ the telescope is in normal adjustment (i.e.,when the final image is at infinity)?
$(b)$ the final image is formed at the least distance of distinct vision $(25 \; cm)$?

The objective and eyepiece of an astronomical telescope are double convex lenses with refractive index $1.5$. When the telescope is adjusted to infinity, the separation between the two lenses is $16 \,cm$. If the space between the lenses is now filled with water and the telescope is again adjusted for infinity, then the present separation between the lenses is: (in $\,cm$)

The length of a telescope is $36 \ cm$. The focal lengths of its lenses can be

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: