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

In a Galilean telescope,if the powers of an objective and eye lens are respectively $+1.25 \, D$ and $-20 \, D$,then for relaxed vision,the length and magnification will be

$A$ lens of large focal length and large aperture is best suited as an objective of an astronomical telescope since :

For a telescope in normal adjustment,the length of the telescope is found to be $27 \, cm$. If the magnifying power of the telescope at normal adjustment is $8$,the focal lengths of the objective and the eyepiece are respectively:

$A$ 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 $(25 \; cm)$?

$A$ telescope has an objective of focal length $100 \ cm$ and an eye-piece of focal length $5 \ cm$. The magnifying power of the telescope is