rest wavelength

The rest wavelength of a source of electromagnetic radiation is the wavelength as observed if at rest in relation to the radiation source, such as is generally the case when a wavelength produced by a laboratory sample is measured. If source and observer are in relative motion, the wavelength observed is termed the observed wavelength. Redshift (z) is calculated as:

     difference in wavelength
z = ——————————————————————————
        rest wavelength

    observed wavelength - rest wavelength
z = —————————————————————————————————————
               rest wavelength

or

                  observed wavelength
rest wavelength = ———————————————————
                        1 + Z

Given the Hubble expansion (the increasing the distance between objects), our meaning of "at rest" here specifically does not mean "at rest within their local Hubble flow", but that the distance between source and observer is not changing, i.e., they are at rest relative to each other.