Astronomy clicker question: Doppler shifted absorption spectra

Astronomy 210, Spring Semester 2009
Cuesta College, San Luis Obispo, CA

Students were asked the following clicker question (Classroom Performance System, einstruction.com) at the end of their learning cycle:

Which absorption spectrum corresponds to a star moving towards the Earth? (The arrows indicate the expected wavelength values for a stationary star of the same type.)

(N.b.: (E) is the "I'm lost, and don't know how to answer this" response.)

Section 30674 (pre-)
(A) : 17 students
(B) : 10 students
(C) : 1 student
(D) : 1 student
(E) : 2 students

This question was asked again after displaying the tallied results with the lack of consensus, with the following results. No comments were made by the instructor, in order to see if students were going to be able to discuss and determine the correct answer among themselves.

Section 30674 (post-)
(A) : 23 students
(B) : 7 students
(C) : 0 students
(D) : 0 students
(E) : 1 student

Correct answer: (A)

For a source moving towards away from an observer, all absorption wavelengths are redshifted ("stretched") to slightly longer values. Note that the rightmost absorption line in spectra (B) is not even shifted at all (likewise, the leftmost absorption line in spectra (D) is unshifted as well).

Pre- to post- peer-interaction gains:
pre-interaction correct = 55%
post-interaction correct = 74%
Hake (normalized) gain <g> = 43%