How do scientists calculate redshift/blueshift in light?

Question

I understand that redshift/blueshift is defined as the change in wavelength of the light divided by the wavelength of the light if the source was not moving. But if what we receive has already redshifted/blueshifted, how do we know that it is not the original wavelength of the light?

Also, once we find out that the light received by us has redshifted/blueshifted, how do we find the original wavelength of the light?

In: Physics

Answer 1

I believe they use the known spectrum for energized hydrogen and helium (known spacing). And look for those lines shifted.