Answer: Doppler effect.
- A Reflection
- B Refraction
- C Doppler effect
- D Interference
Correct answer: C. Doppler effect
Explanation: As the fork approaches and recedes in circular motion, the Doppler effect causes periodic variation in observed frequency.
A standing wave forms from two identical waves travelling in opposite directions and interfering; nodes (always zero displacement) and antinodes (maximum displacement) stay fixed in place, unlike a travelling wave where the whole pattern moves.
Concept context
Wave types, superposition, standing waves, Doppler effect, and sound intensity.