$A$ small telescope has an objective lens of focal length $140 \ cm$ and an eyepiece of focal length $5 \ cm$. The magnifying power of the telescope for viewing a distant object when the telescope is in normal adjustment is . . . . . . .

In a reflecting telescope,a secondary mirror is used to:

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.....$cm$

$A$ Galileo telescope has an objective of focal length $100\,cm$ and magnifying power $50$. The distance between the two lenses in normal adjustment will be......$cm$

The aperture of a telescope is made large,because

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 $\,cm$)