I just had to teach something that I myself have never learned, concerning recessional velocity of galaxies. Awkward!
The question was: "A line in a hydrogen spectrum has a wavelength of 486.1nm when observed in the lab, the same line is observed at 492.3nm from a distant galaxy. Determine the recession velocity of the galaxy and comment on its motion."
While the equation in the associated specification (IGCSE, Edexcel, UK) is:
change in wavelenth / reference wavelength = velocity of galaxy / speed of light
Obviously the left hand term came out less than 1, which I said suggested a red-shifting galaxy, but my tutee thought their teacher had said <1 means its blue-shifting.
Is it as simple as the fact that from the question, the line from the galaxy has a higher wavelength than the lab value, ie: redder, ie red shift?
And therefore, a suitable "comment" on the galaxy's motion would be that it is moving away from the observer?
Thanks in advance for any help.