Physics quiz question: angular magnification of a microscope

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

Cf. Giambattista/Richardson/Richardson, Physics, 2/e Problem 24.37

A microscope has an objective lens and an eyepiece with focal lengths of +1.10 cm and +1.20 cm, respectively. The "barrel length" (distance between the lenses) of the microscope is 14 cm. The angular magnification of this microscope is:
(A) –240×.
(B) –19×.
(C) –11×.
(D) –8.3×.

Correct answer (highlight to unhide): (A)

The angular magnification M of a microscope is given by:

M = –(L – f_e)·N/(f_o·f_e),

where L is the distance between the objective and eyepiece, N = 25 cm is the nominal value for the near point, and f_o and f_e are the focal points of the objective and eyepiece, respectively. Thus with the given values given in the statement of the problem, the angular magnification can be calculated directly:

M = –((14 cm – (+1.20 cm))·(25 cm))/((+1.10 cm)·(+1.20 cm)) = –242.42424242...,

or to two significant figures, the angular magnification of this microscope is –240×, where the negative sign denotes an upside-down image viewed through the microscope.

(Response (B) is –N/(f_o·f_e); response (C) is –L/(f_o·f_e); response (D) is –(N – L)/(f_o·f_e).)

Section 30882
Exam code: quiz02wR4y
(A) : 6 students
(B) : 1 student
(C) : 1 student
(D) : 0 students

Success level: 75%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.67