A Galilean telescope is 27 cm long when focused to form an image at infinity. If the objective has a focal length of 30 cm, what is the focal length of the eyepiece?

Sol. . For the given Galilean telescope, (When the image is formed at infinity) F base 0 = 30 cm, L = 27 cm Since L = f base 0 – |f base e| [Since, concave eyepiece lens is used in Galilean Telescope] ⇒ f base e = f base 0 – L = 30 – 27 = 3 cm