Physics quiz question: object producing real image

Physics 205B Quiz 2, spring semester 2012
Cuesta College, San Luis Obispo, CA

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

An image is located +7.0 cm (to the right) of a converging lens with a focal length of +5.5 cm. The object distance (to the left of the converging lens) is:
(A) +3.1 cm.
(B) +3.9 cm.
(C) +8.0 cm.
(D) +26 cm.

Correct answer (highlight to unhide): (D)

With the image distance d_i = +7.0 cm, and focal length f = +5.5 cm in the thin lens equation, the object distance d_o is:

(1/d_o) = (1/f) – (1/d_i),

such that:

d_o = +25.667 cm, or to two significant figures, +27 cm.

(Response (A) is (1/d_o) = (1/f) + (1/d_i); response (B) is (1/d_o); response (C) is 100/(f + d_i).)

Section 30882
Exam code: quiz02gL4s
(A) : 1 student
(B) : 3 students
(C) : 0 students
(D) : 20 students

Success level: 83%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.67