Physics quiz question: object producing enlarged upright image

Physics 205B Quiz 2, spring semester 2013
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 23.57

A f = +22 cm converging lens creates an upright image that is twice as large as the original object. The object distance is:
(A) +1.1 cm.
(B) +7.3 cm.
(C) +11 cm.
(D) +44 cm.

Correct answer (highlight to unhide): (C)

The linear magnification m equations can yield a relation between the object distance d_o and the image distance d_i:

m = h_i/h_o = –d_i/d_o,

where m = +2 (the positive sign denotes an upright image),

+2 = –d_i/d_o,

–2·d_o = d_i.

Solving for the location of the object d_o by eliminating d_i in the thin lens equation:

(1/d_o) + (1/d_i) = (1/f),

(1/d_o) + (1/(–2·d_o)) = (1/f),

(1/(2·d_o)) = (1/f),

d_o = f/2 = (+22 cm)/2 = +11 cm.

(Response (B) is one-third of the focal length; while response (D) is twice the focal length.)

Section 30882
Exam code: quiz02hYp0
(A) : 0 students
(B) : 3 students
(C) : 23 students
(D) : 7 students

Success level: 70%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.67