The eyepiece of a Galilean telescope is a diverging lens. The focal points F o a

Question

The eyepiece of a Galilean telescope is a diverging lens. The focal points Fo and F’e coincide. In one such telescope, the lenses are a distance d = 32 cm apart and the focal length of the objective is 36 cm. A rhinoceros is viewed from a large distance.
(a) What is the focal length of the eyepiece?

(b) At what distance from the eyepiece is the final image?
(c) Is the final image formed by the eyepiece real or virtual? Upright or inverted? (d) What is the angular magnification? [Hint: the angular magnification is β/α]

Explanation / Answer

From the diagrem the relation between the foval lengths of the lenses is Fo - d = Fe Fo is the focal length of the objective Fe is the focal length of the eye-piece d is the seperation of the lenses 1) Therefore Fe = 36 cm - 32 cm = 4 cm 2) The image of the objective formed at its focus which coisides with the focus of the eye-piece Therefore the final image is formed at infinite 3) As the eye-piece is diverging, the image will be virtual Fo is the focal length of the objective Fe is the focal length of the eye-piece d is the seperation of the lenses 1) Therefore Fe = 36 cm - 32 cm = 4 cm 2) The image of the objective formed at its focus which coisides with the focus of the eye-piece Therefore the final image is formed at infinite 3) As the eye-piece is diverging, the image will be virtual

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