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A Galilean telescope is focused to form the final image at infinity and has a total tube length of 27 cm. The objective lens has a focal length of 30 cm. What is the focal length (in cm) of the eyepiece?
- 3
- 27
- 30
- 57
Correct answer: 3
Solution
A Galilean telescope uses a concave (diverging) lens as the eyepiece. When focused for normal adjustment (image at infinity), the second focal point of the objective coincides with the second focal point of the concave eyepiece. The separation between the lenses equals fₒ - |fₑ|. Given: separation = 27 cm, fₒ = 30 cm. So |fₑ| = 30 - 27 = 3 cm. The focal length of the eyepiece is 3 cm (negative, but magnitude 3).
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