Doppler shifts are always created when the source and receiver are in motion relative to each other.

• A. True
• B. False
• C. Sometimes
• D. Never

Answer: (B)

Doppler shifts depend on motion along the direction the wave travels. Only the component of relative velocity that lies along the beam path changes how quickly wavefronts are encountered, which shifts the frequency. If the motion is purely sideways (perpendicular to the beam) or if the source and receiver move together so the distance along the beam doesn’t change, there’s no change in the encounter rate, and no Doppler shift occurs. So, motion of the source and receiver in general does not guarantee a Doppler shift; it occurs only when there is a radial component of motion toward or away from the beam. This is why the statement is false.