Question

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?

Answer 1

Both receding, B faster than A.

Step-by-step explanation:

The doppler effect relates to the fact that if a source is moving, the value we will obtain by measuring its wavelength could be different from the value if it were at rest (there are very particular cases, classical and relativistic, where the masurement remains the same).

Generally, if the wavelength we measure is longer the object is receding from us (since the wave stretches) and if it is shorter the object is coming to us (the wave compresses), and the bigger the difference the faster it does so.

For this example then, we conclude that both clouds are receding from us (getting farther away), Cloud B faster than Cloud A.