Which equation relates Doppler frequency shift to velocity?

• A. Poiseuille's law
• B. Bernoulli's principle
• C. Doppler equation
• D. Reynolds number

Answer: (C)

The main concept being tested is how motion affects the frequency we observe, known as the Doppler effect. The Doppler equation is the one that links the Doppler frequency shift directly to velocity along the line of sight between source and observer. In general, the observed frequency f' differs from the emitted frequency f0 in a way that depends on how fast the source or observer is moving toward or away from each other and on the wave speed in the medium. In many practical forms, especially in ultrasound contexts, the Doppler shift is written to show this direct connection to velocity, for example f_d = 2 f0 v cos θ / c, which makes the relationship explicit. The other formulas describe fluid flow or pressure–velocity relationships rather than frequency changes due to motion, so they don’t describe Doppler frequency shifts.