Which physical concept is used to measure velocity of moving reflectors via Doppler shifts?

• A. Huygen's Principle
• B. Doppler Effect
• C. Echo Ranging
• D. Snell's Law

Answer: (B)

Doppler effect. When a wave hits a moving reflector, the frequency of the reflected wave shifts in proportion to the velocity component along the line of sight. This Doppler shift allows you to infer how fast the object is moving toward or away from the observer. For nonrelativistic speeds, the shift in the reflected frequency is approximately Δf ≈ 2v/λ, where v is the radial velocity and λ is the wavelength of the transmitted wave. The factor of two comes from the wave being shifted on the way to the moving surface and again on the way back. By measuring this frequency change, you can calculate the velocity of the moving reflector.

Huygen's principle relates to constructing wavefronts, not measuring velocity from frequency changes. Snell's Law governs refraction at boundaries, and echo ranging uses timing to gauge distance, not velocity via Doppler shifts.