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.


1 Answer 1


And answers own question:

After z is measured, the distinction between redshift and blueshift is simply a matter of whether z is positive or negative. For example, Doppler effect blueshifts (z < 0) are associated with objects approaching (moving closer to) the observer with the light shifting to greater energies. Conversely, Doppler effect redshifts (z > 0) are associated with objects receding (moving away) from the observer with the light shifting to lower energies.

Frustrating. I originally thought the left hand term would be significant by its sign (positive or negative) but got flustered and started thinking it would be more or less than 1. Embarrassing.


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