Doppler effect for moving source toward observer: if the source is stationary (v_s = 0), what is the observed frequency?

• A. The observed frequency increases
• B. The observed frequency decreases
• C. The observed frequency equals the emitted frequency f
• D. The observed frequency becomes undefined

Answer: (C)

The Doppler effect comes from the changing spacing of wavefronts due to relative motion between source and observer. If the source isn’t moving, it emits waves at a steady rate and the wavefronts travel through the medium without being compressed or stretched toward or away from the observer. Because there’s no motion of the source toward or away from the observer, the observer encounters the wavefronts at the same rate they’re emitted, so the observed frequency stays the same as the emitted frequency. In short, the observed frequency equals the emitted frequency.