Double Speed Doubles the Shift Ratio - Observational Astrophysics: Distance, Flux, Transits, and Spectra Step by Step

Changing only radial speed changes the exact signed ratio. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.

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Only radial speed doubles

The radial speed changes to 2 meters per second while light speed stays 6.

\frac{v}{c}=\frac{2}{6}

The shift ratio becomes one third

The exact checked ratio is 1/3, so the contrast is proportional.

\Delta\lambda/\lambda=\tfrac{2}{6}=\tfrac{1}{3}