The Doppler effect gives us information on

• A. Distance to the object
• B. Mass of the object
• C. Rotation speed
• D. The velocity of an object towards or away from us

Answer: (D)

Motion along the line of sight changes the observed wavelength of light: approaching objects appear blueshifted (shorter wavelengths) and receding ones appear redshifted (longer wavelengths). By comparing the observed wavelength to the known rest wavelength of a spectral line, we can determine the radial velocity—the speed at which the object is moving toward or away from us. This is the essence of what the Doppler effect tells us. It doesn’t directly give distance, mass, or rotation speed. Distance and mass require other methods or models, and rotation speed can only be inferred from Doppler effects across extended objects (through broadening and variations in velocity across the source), whereas the basic shift alone specifies only line-of-sight motion.