AQA A-Level Physics Paper 1, June 2023: Question 15

✅ Correct Answer

B — They have a constant phase relationship.

Why it's not correct (making it the right choice for the "NOT correct" stem): Two independent progressive waves travelling in opposite directions to form a standing wave do not have a constant phase relationship throughout the medium. Their phase difference varies continuously with position because they are travelling in opposite directions.

💡 Key Knowledge

• Standing Waves are formed by the superposition of two progressive waves of the same frequency and speed (and therefore same wavelength) travelling in opposite directions.
• Coherence vs Superposition: While coherent sources are needed to produce observable interference patterns (like Young's slits), the specific requirement for a textbook standing wave setup is identical frequency, amplitude, and speed travelling oppositely.

🧠 Exam Technique

Always double-check negative stems like "not correct". Underline or circle the word NOT in the exam paper so you don't accidentally pick a true statement. Options A, C, and D are true properties of the component waves, leaving B as the false statement.

❌ Common Errors

Students often confuse the conditions for standard interference (which requires a constant phase difference / coherence) with the superposition forming a standing wave on a restricted string or pipe. Because the two waves move in opposite directions, the phase relationship between them changes constantly from point to point.