Physics quiz question: real vs. virtual object for second lens

Physics 205B Quiz 2, Spring Semester 2011
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Figure 24.2.

An object is placed 24.0 cm to the left of a converging lens with focal length +8.0 cm. What is the minimum separation distance between this first lens and a second diverging lens with focal length –4.0 cm such that the intermediate image from the first lens will not be a virtual object for the second lens?
(A) 8 cm.
(B) 12 cm.
(C) 20 cm.
(D) 16 cm.

Correct answer: (B)

For the first lens with focal length f_1 = +8.0 cm, the object distance is p_1 = 24.0 cm, such that from the thin lens equation, q_1 = +12 cm, meaning that the first lens produces a real image 12 cm to the right (behind) it. For this intermediate real image to be considered a real object by the second lens, it must be located to the left (in front) of the second lens. Thus the second lens must be located at a minimum of 12 cm to the right (behind) the first lens.

Student responses
Section 30822
(A) : 1 student
(B) : 2 students
(C) : 2 students
(D) : 3 students

Success level: 25%
Discrimination index (Aubrecht & Aubrecht, 1983): 0