AQA A-Level Physics Paper 1, June 2022: Question 18

✅ Correct Answer: C

It oscillates with a time period of 1.5 ms.

The time period T is the reciprocal of frequency ( T = 1 / f ). Calculating 1 / 660 Hz gives approximately 0.00151 s , which is 1.5 ms .

💡 Key Knowledge

• Wave Speed vs Particle Speed: Waves transfer energy through a medium, but individual particles oscillate back and forth about an equilibrium position—they do not travel along with the wave at the wave speed.
• Time Period Formula: T = 1 / f
• Wavelength Calculation: λ = c / f = 330 / 660 = 0.5 m (or 50 cm).

🧠 Exam Technique

Read through each option systematically and eliminate false statements using core definitions:

• Check particle vs wave velocity.
• Calculate wavelength to test phase relationships (particles are in phase separated by whole numbers of wavelengths, λ = 0.5 m or 50 cm , not 25 cm).
• Quickly compute T = 1 / f to verify numerical options.

❌ Common Errors

• Confusing A with particle motion: Believing that particles travel through the medium at the wave speed ( 330 m s⁻¹ ).
• Phase misconceptions (Option B): Failing to calculate wavelength ( 50 cm ) and incorrectly assuming a separation of 25 cm ( λ / 2 ) puts particles in anti-phase, not in phase.
• Frequency misinterpretation (Option D): Thinking frequency represents directional changes rather than complete oscillation cycles (though a particle changes direction twice per cycle, the frequency itself is 660 Hz , meaning 660 complete cycles per second, resulting in 1320 directional changes per second).