SAT Subject Physics Practice Question 627: Answer and Explanation
8. Astronomers can tell whether a star is approaching or receding from the earth by the
A. absorption spectra of the star.
B. Doppler shift of the starlight.
C. temperature of the star.
D. brightness of the starlight.
E. thermal signature of the star.
Correct Answer: B
The correct answer is (B). Stars are a source of light waves. As such, the waves approach the earth as fronts. When a star is approaching the earth the wave fronts reach us at an increasing rate. When a star is receding from us, the wave fronts reach us at a lower rate. In both cases a frequency change (Doppler shift) can be detected.