Four lenses of focal length $\pm 15\; cm$ and $\pm 150 \;cm$ are available for making a telescope. To produce the largest magnification,the focal length (in $cm$) of the eyepiece should be

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

$A$ telescope has an objective lens of focal length $200 \, cm$ and an eyepiece with focal length $2 \, cm$. If this telescope is used to see a $50 \, m$ tall building at a distance of $2 \, km$,what is the height of the image of the building formed by the objective lens in $cm$?

The minimum magnifying power of a telescope is $M$. If the focal length of its eye lens is halved, the magnifying power will become:

$A$ Cassegrain telescope uses two mirrors of radii of curvature $25 \ cm$ and $16 \ cm$ separated by a distance of $2.5 \ cm$. The position of the final image of an object at infinity is

The magnifying power of a telescope can be increased by