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Which of the following statements about wave superposition and sound waves are correct? (A) When two waves of different frequencies f1 and f2 superimpose at a point, the intensity varies with frequency |f1 - f2|. (B) When an observer moves, the relative speed of sound waves with respect to the observer changes; when the source moves, the wavelength of the sound wave changes. (C) When a high-pressure compression pulse reaches the open water surface in a resonance column, it reflects as a low-pressure rarefaction pulse due to a phase change of pi. (D) When a tuning fork is struck by a hammer, a transverse wave sets up in the prong while a longitudinal wave sets up in the stem (handle).

1. (A), (B) and (C) only
2. (A), (B) and (D) only
3. (B), (C) and (D) only
4. (A), (C) and (D) only

Correct answer: (A), (B) and (D) only

Solution

A: Beat frequency = |f1-f2|, intensity varies at this frequency — CORRECT. B: Observer motion changes relative wave speed; source motion changes wavelength — CORRECT. C: In a resonance column, the water surface is a closed end (displacement node, pressure antinode). A compression pulse reflects as a compression at a closed end (no phase change for pressure). Statement C is INCORRECT. D: Prongs vibrate transversely, stem vibrates longitudinally — CORRECT. Correct statements: A, B, D.

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