Physics quiz question: comparing radio wave speeds

Physics 205B Quiz 1, spring semester 2013
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Conceptual Question 23.7

"Under the radio towers on Capitol Hill"
Matthew Rutledge
flic.kr/p/5JTuuF

FM radio station KCBX broadcasts at 90.1 MHz, while AM radio station KVEC broadcasts at 920 kHz[*]. Radio signals from station __________ travel with a faster speed in air.
(A) KCBX.
(B) KVEC.
(C) (There is a tie.)
(D) (Not enough information given.)

[*] radio-locator.com/cgi-bin/locate?select=city&city=San+Luis+Obispo&state=CA.

Correct answer (highlight to unhide): (C)

Wave speed v depends on the properties of the medium. Frequency f depends on the properties of the source. These two parameters can be varied independently of each other.

The wavelength λ is the parameter dependent on both of the independent parameters:

λ = v/f.

The radio waves from these two stations must have the same speed v, as they travel through the same medium. However, they can have different f frequencies (as this parameter depends only the wave source), and thus must also have different λ wavelengths (as this parameter depends on both the wave source and the properties of the medium).

Section 30882
Exam code: quiz01b3Es
(A) : 6 students
(B) : 9 students
(C) : 19 students
(D) : 1 student

Success level: 51%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.05