Cuesta College, San Luis Obispo, CA
The Lick Observatory telescope[*] has a 1,763 cm focal length objective lens with a 3.2 cm focal length eyepiece. This eyepiece is then replaced by a 1.6 cm focal length eyepiece. As a result of replacing eyepieces, the distance between the objective lens and the eyepiece must __________ for the telescope to remain in focus.
(B) remain the same.
(D) (Not enough information given.)
Correct answer (highlight to unhide): (C)
The distance between the objective lens and eyepiece for a telescope (focused on a distant object) is merely the sum the of the objective lens focal length fo and the eyepiece focal length fe, which originally was:
fo + fe = 1,763 cm + 3.2 cm = 1,766 cm.
Then replacing the eyepiece with shorter 1.6 cm focal length lens (which would increase its angular magnification) would result in a shorter distance between the objective lens and eyepiece:
fo + fe = 1,763 cm + 1.6 cm = 1,765 cm.
Thus the length of the telescope must be shortened by approximately 1 cm in order to remain in focus.
Since the intermediate real image is located a distance of fo from the objective lens, and a distance of fe from the eyepiece, then the total distance from the objective lens to the eyepiece is just fo + fe.)
Sections 30882, 30883
Exam code: quiz02f3MA
(A) : 9 student
(B) : 3 students
(C) : 29 students
(D) : 0 students
Success level: 71%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.50