Doppler Effect for Sound

Steps:

• The Doppler effect shifts the observed frequency due to relative motion between sound source and stationary observer.
• As the train approaches, sound waves compress ahead of it, shortening wavelength and increasing frequency above f.
• As the train recedes, waves stretch behind it, lengthening wavelength and decreasing frequency below f.
• The train's constant speed ensures the frequency change depends only on approach or recession direction.

Why B is correct:

• B aligns with the Doppler formula f' = f (v / (v - vs)) for approach (f' > f) and f' = f (v / (v + vs)) for recession (f' < f), where v is sound speed and vs is source speed.

Why the others are wrong:

• A: Decreasing frequency on approach contradicts wave compression.
• C: Reverses the Doppler shift directions.
• D: Ignores recession, which decreases frequency.

Final answer: B