Physics quiz question: wrong-way telescope

Physics 205B Quiz 2, spring semester 2011
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Comprehensive Problem 24.71.

A telescope has an objective lens and an eyepiece with focal lengths of +155 cm and +2.0 cm, respectively. If a Physics 205B student looks through this telescope the wrong way (that is, with the eye placed at the objective), the angular magnification would be:
(A) –13.
(B) –0.49.
(C) –0.013.
(D) –0.0065.

Correct answer (highlight to unhide): (C)

When looking through the telescope the right way, the angular magnification will be given by:

M = –f_o/f_e,

where f_o should be the longer focal length of the two lenses used in a proper telescope, such that:

M = –(+155 cm)/(+2.0 cm) = –78.

Looking through this telescope the wrong way would be the effectively switching the roles of the objective and eyepiece lens, resulting in the inverse of this magnification, which would be:

M = –(+2.0 cm)/(+155 cm) = –0.013.

Response (A) is using the microscope angular magnification equation –(L – f_e)·N)/(f_o·f_e) (where L = f_o + f_e, as with telescopes); response (B) is 1/f_e – 1/f_o; response (C) is 1/(f_o – f_e).

Student responses
Section 30822
(A) : 1 student
(B) : 0 students
(C) : 7 students
(D) : 0 students

Success level: 88%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.33