Cuesta College, San Luis Obispo, CA
Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 25.3.
Two coherent radio transmitters broadcast out of phase signals with the same wavelength. A radio antenna receives a constructive interference signal from these transmitters, and is located 1.85 km away from one transmitter, and 2.25 km away from the other transmitter. The longest possible wavelength of the transmitters is:
(A) 0.20 km.
(B) 0.27 km.
(C) 0.40 km.
(D) 0.80 km.
Correct answer: (D)
The path-length difference ∆l = (2.25 km - 1.85 km) = 0.40 km. Since the two sources are out-of-phase, then the constructive interference condition becomes:
∆l = (m + 1/2)·λ,
where the longest possible wavelength is given by:
∆l/(m + 1/2) = λ = 0.80 km, by choosing m = 0.
Response (A) is ∆l/2; response (B) is (2/3)·∆l, which is the second-longest possible wavelength; response (C) is delta(l).
Student responses
Section 30822
Exam code: quiz03F3ld
(A) : 1 student
(B) : 2 students
(C) : 4 students
(D) : 0 students
Success level: 0%
Discrimination index (Aubrecht & Aubrecht, 1983): 0
In the subsequent lecture, students were instructed to re-do this problem in small groups on whiteboards, solving for the longest possible wavelength (0.80 km), but also to find the second-longest possible wavelength as well (0.27 km).
110314-1220612http://www.flickr.com/photos/waiferx/5526922797/
Originally uploaded by Waifer X
No comments:
Post a Comment