## 20150222

### 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 = –(LfeN/(fo·fe),

where L is the distance between the objective and eyepiece, N = 25 cm is the nominal value for the near point, and fo and fe 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/(fo·fe); response (C) is –L/(fo·fe); response (D) is –(N – L)/(fo·fe).)

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