(a) Draw a ray diagram depicting the formation of the image by an astronomical telescope in normal adjustment.
(b) You are given the following three lenses. Which two lenses will you use as an eyepiece and as an objective to construct an astronomical telescope? Give reason.

(a) Ray diagram of astronomical telescope,
(b) In an astronomical telescope,
The eyepiece has greater power while a smaller aperture. and, the objective has lower power with a longer aperture so, by using 4 as objective and L3 are eyepiece, the best possible magnification,
$\mathrm{m}=\frac{{\mathrm{P}}_{\mathrm{e}}}{{\mathrm{P}}_0}=\frac{10}{3}=3.33$ can be achieved.