Physics quiz question: telescope angular magnification

Physics 205B Quiz 2, spring semester 2012
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 24.51

A telescope has an eyepiece with a focal length of +2.0 cm, and the distance between the objective lens and eyepiece is 24.0 cm. The angular magnification of this telescope is:
(A) –48×.
(B) –13×.
(C) –12×.
(D) –11×.

Correct answer (highlight to unhide): (D)

The far focal point of the objective lens coincides with the near focal point of the eyepiece, such that the lens-to-lens distance L of the telescope is the sum of the objective and eyepiece focal lengths:

L = f_objective + f_eyepiece,

24.0 cm = f_objective + 2.0 cm,

where the objective lens focal length is then f_objective = +22.0 cm. Thus the angular magnification is given by:

M = –f_objective/f_eyepiece = –(22.0 cm)/(2.0 cm) = –11×.

(Response (A) is –f_objective·f_eyepiece; response (B) is (25.0 cm)/f_eyepiece; and response (C) is (24.0 cm)/f_eyepiece.)

Section 30882
Exam code: quiz02gL4s
(A) : 0 students
(B) : 4 students
(C) : 6 students
(D) : 14 students

Success level: 58%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.57