Redshift

Redshift measures how much a light spectrum has been stretched so that an emitted wavelength is seen at a longer wavelength by an observer. The formula compares the change in wavelength to the original emitted wavelength, which makes z a dimensionless fractional shift rather than an absolute distance. This works because many astrophysical effects—especially the expansion of the Universe, and sometimes motion away from the observer—change all spectral lines by the same proportion, letting astronomers compare different objects on a common scale.

Use this when you know a spectral line’s rest wavelength and its observed wavelength and want to quantify the shift. It is especially useful in cosmology, galaxy spectroscopy, and radial-velocity studies where line positions reveal expansion or motion.

• Using the observed wavelength in the denominator instead of the emitted (rest) wavelength
• Forgetting that redshift is a ratio, so the units cancel and z has no units
• Mixing up redshift with wavelength difference alone and reporting only λ_obs - λ_emit