We are measuring the spectra of two hydrogen gas clouds. The laboratory frame wavelength of one hydrogen line is 656.2 nm. Cloud A's emission line wavelength is 660.1 nm and Cloud B's emission line wavelength is 670.1 nm. What can we conclude about these clouds?
A) They are both approaching us, and Cloud B is approaching faster than Cloud A.
B) They are both receding from us, and Cloud B is receding faster than Cloud A.
C) They are both approaching us, and Cloud A is approaching faster than Cloud B.
D) They are both receding from us, and Cloud A is receding faster than Cloud B.

Question

Quizplus · Accepted Answer

The laboratory frame wavelength of the hydrogen line is the rest wavelength in vacuum. If a gas cloud is moving towards us, the observed wavelength of its spectral lines will be shorter (blueshifted) than the rest wavelength. If a gas cloud is moving away from us, the observed wavelength of its spectral lines will be longer (redshifted) than the rest wavelength.

Cloud A's emission line at 660.1 nm has an observed wavelength that is longer than the rest wavelength of 656.2 nm, indicating that Cloud A is receding from us. Similarly, Cloud B's emission line at 670.1 nm has an observed wavelength that is longer than the rest wavelength, indicating that Cloud B is also receding from us.

Since the observed wavelength of Cloud B is longer than that of Cloud A, we can conclude that Cloud B is receding faster than Cloud A. Therefore, the correct choice is B: They are both receding from us, and Cloud B is receding faster than Cloud A.

We Are Measuring the Spectra of Two Hydrogen Gas Clouds