Doppler Effect for Moving Source

Steps:

• Stationary source emits sound at frequency f, observed as f by stationary observer.
• When source moves away, it stretches the wavefronts, increasing wavelength toward the observer.
• Longer wavelength means lower observed frequency, per f' = f * (v / (v + v_s)), where v is sound speed and v_s is source speed away.
• Source's emitted frequency remains f, unchanged by its motion.

Why D is correct:

• Observed frequency decreases because the source's motion away from the observer causes a Doppler shift, lengthening the perceived wavelength.

Why the others are wrong:

• A: Source frequency stays f (not higher); observed frequency is lower, not higher.
• B: Source frequency remains f (not lower); only observed frequency decreases.
• C: Source frequency is unchanged at f, regardless of motion.

Final answer: D