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

Write a note on Cassegrain telescope.

When a telescope is adjusted for parallel light,the distance of the objective from the eyepiece is found to be $80 \,cm$. The magnifying power of the telescope is $19$. The focal lengths of the lenses are

In normal adjustment,for a refracting telescope,the distance between the objective and the eyepiece is $30\,cm$. The focal length of the objective,when the angular magnification of the telescope is $2$,will be $.....\,cm$.

$A$ telescope has an objective lens of focal length $144\, cm$ and an eyepiece of focal length $6\, cm$. Find the magnifying power of the telescope for normal adjustment.

In an astronomical telescope in normal adjustment,a straight black line of length $L$ is drawn on the inside part of the objective lens. The eyepiece forms a real image of this line. The length of this image is $I$. The magnification of the telescope is