An astronomical telescope has an objective of focal length 20 cm and an eyepiece of focal length 4.0 cm. The telescope is focused to see an object 10 cm from the objective. the final image is formed at infinity. Find the length of the tube and the angular magnification produced by the telescope.

Explanation

As the object distance 10 km is much larger thanthe focal lkength 200 cm, the first imae is formed amost at the focuis of the objective. It is thus 200 cm from the objective. This image acts as the object for the eyepiece. To ge the fiN/Al image at infinity, this first image sould be at the first focus of the eyepiece.The length of the tube is therefore 200 cm+4cm=204 cm. The angular magnification in this case
`m=-f_0/f_e=200/4=-50`.