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

Here we treat the line on the objective as the object and the eyepiece as the lens. Hence u=−(fo+fe) and f=fe Now  1v−1−(fo+fe)=1feSolving we get  v=(fo+fe)fefoMagnification  =vu=fefo=Image sizeObject size=lL∴ Magnification of telescope in normal adjustment =fofe=Ll