List of top Physics Questions on Optical Instruments asked in TS EAMCET

Keeping the length of an astronomical telescope constant in normal adjustment, to increase its magnification from 4 to 9, the focal length of the eyepiece is to be decreased by 10 cm. The length of the telescope is

In an astronomical telescope of $135\text{ cm}$ length kept in normal adjustment, if the difference between the focal lengths of the objective and eyepiece is $125\text{ cm}$, then the magnification of the telescope is: