AQA AS Level Physics Paper 2, June 2023: Question 6

1 mark · Medium difficulty · Multiple Choice

Determine the phase difference between microwaves from two coherent sources at a point of zero reading that is equidistant from both sources.

Practise this question

Question

Multiple choice question 06 asking for the phase difference between microwaves from two coherent sources S1 and S2 at a detector giving a zero reading when equidistant from both sources. Four options are provided: A (zero), B (1.6 rad), C (3.1 rad), and D (6.3 rad).
Question text

06 S1 and S2 are coherent sources of microwaves that produce waves of the same amplitude.

A microwave detector gives a zero reading when placed at a point that is the same

distance from S1 and S2.

What is the phase difference between microwaves from S1 and S2 at the detector?

[1 mark]

A zero

B 1.6 rad

C 3.1 rad

D 6.3 rad

Mark scheme

Show the mark scheme Mark scheme for question 06 indicating that the correct answer is C, corresponding to 3.1 rad.

06 C 3.1 rad

How to answer it

Microwave Interference & Phase Difference

What this question tests

This question assesses your understanding of wave superposition, path difference, destructive interference, and the relationship between path difference and phase difference in radians. It tests whether you can connect a zero reading (complete cancellation) to the correct fractional wavelength condition and convert it into angular measure.

Question 06

Determining Phase Difference for Destructive Interference

✅ Correct Answer: C (3.1 rad)

The correct option is C because a zero reading means total destructive interference occurs. Since the path difference is zero (equal distance from both sources), the phase difference must stem from an initial phase change at the sources (or a half-cycle phase shift equivalent to 3.1 rad / pi radians for destructive interference).

💡 Key Knowledge

  • Path Difference: Difference in distance travelled by two waves from coherent sources to a point. Here, path difference = 0.
  • Destructive Interference: Occurs when waves arrive out of phase, leading to cancellation (zero amplitude).
  • Phase Relation: Phase difference (phi) = (2 * pi / wavelength) * path difference. For destructive interference without path difference, sources must have a phase difference of odd multiples of pi radians (e.g., pi rad).
  • Numerical Value: pi radians = 3.14 rad, which rounds to 3.1 rad.

🧠 Exam Technique

Don't automatically reflex-guess "zero" just because the distances are equal! Read the stem carefully: a zero reading means destructive interference. Destructive interference requires a phase difference of 180 degrees or pi radians (3.1 rad), not zero.

❌ Common Errors

  • Assuming equal distance means zero phase difference: Many students saw "same distance from S1 and S2" and immediately selected A (zero), forgetting that a zero detector reading requires the waves to arrive out of phase (destructively interfere).
  • Unit confusion: Confusing degrees with radians or miscalculating the numerical value of pi.
Mark Scheme Note: 1 mark awarded for selecting option C.

Topics

Physics · 3.3 Waves

Question and mark scheme from the AQA AS Level Physics examination, Paper 2, June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.