For a telescope,the focal length of the objective lens is $15\, cm$ and the focal length of the eyepiece is $10\, mm$. If the tube length is $16\, cm$,find the magnification.

The magnifying power of an astronomical telescope is $8$ and the distance between the two lenses is $54 \, cm$. The focal lengths of the eye lens and the objective lens will be respectively:

The magnifying power of a refracting type of astronomical telescope is $m$. If the focal length of the eyepiece is doubled,then the magnifying power will become:

If the tube length of an astronomical telescope is $105 \, cm$ and the magnifying power is $20$ for normal setting,calculate the focal length of the objective in $cm$.

The magnifying power of an astronomical telescope is $8$. If the distance between the two lenses is $54 \, cm$,find the focal lengths of the eyepiece and the objective lens.

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.....$cm$