AQA AS Level Physics Paper 2, June 2024: Question 30

✅ Correct Answer: D

They must be polarised in the same plane.

For two waves to undergo complete constructive interference and form a stationary wave with maximum possible amplitude (an antinode), their oscillations must be aligned in the exact same plane. If they were polarised at right angles (perpendicular planes), they could not component-add to reinforce each other in the manner required.

💡 Key Knowledge

• Stationary Waves: Formed by the superposition of two progressive waves of the same frequency and amplitude travelling in opposite directions.
• Antinodes: Points of maximum amplitude where constructive interference occurs continuously.
• Polarisation: Restriction of vibration to a single plane, which only applies to transverse waves.

🧠 Exam Technique

Eliminate options systematically:

• A is false: Longitudinal waves cannot be polarised because their oscillations are parallel to energy propagation.
• B is misleading: While they originate from the same source (making them coherent), coherence alone does not dictate antinodal behaviour if polarisation planes are misaligned.
• C is false: Stationary waves require interfering waves of the same frequency.

❌ Common Errors

Students frequently guess Option B because they associate wave superposition heavily with the general condition of coherence. Examiners note that candidates often forget that transverse wave components must also share a vibration plane to constructively interfere to maximum effect.