Physics quiz question: simple magnifier angular magnification

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

A Physics 205B student would like to use a lens with a focal length of +5.0 cm as a simple magnifier. Assume the student's eye is held close to the lens. What is the angular magnification if the final image is at infinity?
(A) +4.2⨉.
(B) +5.0⨉.
(C) +6.3⨉.
(D) +20⨉.

Correct answer (highlight to unhide): (B)

The angular magnification of a converging lens, used as a simple magnifier (and a relaxed eye) is given by:

M = N/f,

where N is the nominal near point of 25 cm. Thus the angular magnification is:

M = N/f = (0.25 m)/(+5.0 cm) = +5.0⨉.

(Response (A) is incorrectly (and inappropriately) solving for the image distance d_i for an object placed at d_o = +25 cm with a sign error (1/d_i) = 1/(f) + (1/d_o); response (C) is correctly (but inappropriately) solving for an image distance d_i for an object placed at d_o = +25 cm; response (D) is 100/f.)

Sections 30882, 30883
Exam code: quiz02P0wR
(A) : 0 students
(B) : 31 students
(C) : 2 students
(D) : 1 student

Success level: 91%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.14