Home Lesson for Students
   
When a light source (like a hydrogen atom)  is moving towards you very fast, the light waves are compressed and the light appears more blue to you that it would if it were standing still.  If the light source is moving away from you, the waves are stretched apart and appear more red to you than they would if they were standing stilll?

 

How would the spectra of a hydrogen atom that is moving towards you look different than the one shown below?
How would the spectra of a hydrogen atom that is moving away from  you look different than the one shown below?
Now imagine a cloud of hydrogen - some of the atoms are moving towards you, some are moving away from you and some are (at least for a instant) not moving either way.  If you were to look at the spectra from all of those molecules together - what might you see?

 

Hydrogen Emission Spectra