An opera glass (Galilean telescope) measures $9 \, cm$ from the objective to the eyepiece. The focal length of the objective is $15 \, cm$. Its magnifying power is

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 $\,cm$)

In an astronomical telescope,the focal length of the objective lens is $100 \, cm$ and of the eyepiece is $2 \, cm$. The magnifying power of the telescope for the normal eye is:

$A$ Cassegrain telescope uses two mirrors as shown in the figure. Such a telescope is built with the mirrors $20 \; mm$ apart. If the radius of curvature of the large mirror is $220 \; mm$ and the small mirror is $140 \; mm$,where will the final image of an object at infinity be?

An observer looks at a tree of height $15 \ m$ with a telescope of magnifying power $10$. To him,the tree appears

An astronomical telescope has an objective and an eyepiece of focal lengths $40\, cm$ and $4\, cm$ respectively. To view an object $200\, cm$ away from the objective,the lenses must be separated by a distance of.....$cm$.