If a hydrogen emission line appears at 656.2 nm in the laboratory, what can we say about a cloud of hydrogen gas, with an emission line at 680.2 nm?
A) It is receding away from us.
B) The location of the emission line has nothing to do with speed.
C) It is neither receding nor approaching us.
D) It is approaching us.

Question

Quizplus · Accepted Answer

The wavelength of the emission line is longer than the laboratory value, which means that the cloud is receding away from us. This is known as redshift.

If a Hydrogen Emission Line Appears at 656