Physics quiz question: telescope eyepiece focal length

Physics 205B Quiz 2, spring semester 2018
Cuesta College, San Luis Obispo, CA

An Orion Observer telescope[*] uses an objective lens with a focal length of +70.0 cm. If the magnification obtained is –28⨉, then the focal length of the eyepiece is:
(A) +0.40 cm.
(B) +0.88 cm.
(C) +2.5 cm.
(D) +42 cm.

[*] telescope.com/catalog/product.jsp?productId=118162.

Correct answer (highlight to unhide): (C)

The angular magnification of a telescope is given by:

M = –f_objective/f_eyepiece,

and so the focal length of the eyepiece is then:

f_eyepiece = –f_objective/M,

f_eyepiece = –(+70.0 cm)/(–28) = +2.5 cm.

(Response (A) is –M/f_objective; response (B) is –L·N/(f_objective + N), which is an inappropriate use of the microscope angular magnification equation; response (D) is f_objective – M.)

Sections 30882, 30883
Exam code: quiz02P0wR
(A) : 1 student
(B) : 2 students
(C) : 30 students
(D) : 1 student

Success level: 88%
Discrimination index (Aubrecht & Aubrecht, 1983): 0