Doppler effect causes frequency shift for moving sound source

Steps:

• Vehicle approaches observer, compressing sound waves ahead, increasing frequency.
• As vehicle passes, waves stretch behind, decreasing frequency.
• This change occurs due to relative motion between source and observer.
• Matches Doppler effect for sound waves.

Why D is correct:

• Doppler effect formula $f' = f \frac{v \pm v_o}{v \mp v_s}$ (v = sound speed, observer speed, source speed) predicts higher frequency approaching, lower receding.

Why the others are wrong:

• A: Diffraction bends waves around obstacles, not frequency change from motion.
• B: Interference combines waves for patterns, unrelated to source movement.
• C: Polarization aligns transverse waves like light, inapplicable to longitudinal sound waves.

Final answer: D