Which phenomenon allows us to determine the velocity of an object along our line of sight by shifts in wavelength?

• A. Parallax
• B. Gravitational lensing
• C. Interstellar extinction
• D. Doppler effect

Answer: (D)

Shifts in wavelength reveal motion along our line of sight through the Doppler effect. When an object moves toward us, its light is blueshifted (wavelengths get shorter); when it moves away, it is redshifted (wavelengths get longer). By measuring how much the spectral lines have shifted from their known rest wavelengths, we can determine the radial (line-of-sight) velocity. In the simple case, the velocity is approximately the speed of light times the fractional shift Δλ/λ0 (or using z = Δλ/λ0, v ≈ z c for small speeds; a full relativistic formula is used for high speeds).

Other phenomena don’t give this velocity information in the same way. Parallax tracks apparent position shifts over time to infer distance, not velocity from wavelength changes. Gravitational lensing bends light and can create multiple images or magnify objects, but it doesn’t produce the wavelength shifts that encode radial velocity. Interstellar extinction dims and reddens light due to dust, not due to motion along our line of sight.