AQA AS Level Physics Paper 2, June 2022: Question 11

1 mark · Medium difficulty · Multiple Choice

Identify the correct row in the table that explains the observation when a narrow beam of light is incident on a Polaroid sheet which is then rotated by 90 degrees.

Practise this question

Question

A multiple-choice question featuring a diagram of a narrow beam of light passing through a Polaroid sheet. The question asks to identify the correct row from a table with columns 'Nature of incident beam' and 'Action of Polaroid material as it is rotated' that corresponds to the intensity dropping to zero when rotated by 90 degrees.
Question text

11 A narrow beam of light is incident on a sheet of Polaroid material. The intensity of the

transmitted beam is a maximum.

The Polaroid sheet is rotated about the beam by 90° and the intensity of the transmitted

beam decreases to zero.

Which row explains this observation?

[1 mark]

Nature of incident beam Action of Polaroid material as it is rotated

A unpolarised polarises the incident beam

B unpolarised absorbs the incident beam

C polarised absorbs the incident beam

changes the plane of polarisation of

D polarised

the incident beam

Mark scheme

Show the mark scheme The mark scheme indicates that the correct answer is option C, which states the incident beam is polarised and the Polaroid material absorbs it.

11 C (AO1) polarised absorbs the incident beam

How to answer it

Polarisation and Polaroid Filters

AQA AS Level Physics • Waves

What this question tests

This question assesses your understanding of the transverse nature of light waves, specifically how Polaroid filters interact with polarised versus unpolarised light. You must understand how rotating a filter relative to an incident beam affects the transmitted intensity and correctly interpret the optical mechanisms involved.

Question 1.1

Multiple-Choice Analysis [1 Mark]

✅ Correct Answer: C

The correct option is C: The incident beam is polarised and the Polaroid material absorbs the incident beam as it rotates to 90°.

Mark Scheme: C (AO1)

💡 Key Knowledge

  • Polarisation only occurs with transverse waves, where oscillations are restricted to a single plane.
  • If an unpolarised beam passes through a Polaroid sheet, rotating the sheet by 90° will decrease the transmitted intensity, but never to zero (intensity drops to 50% of the initial value).
  • For the intensity to drop all the way to zero, the incident beam must already be plane-polarised. When the transmission axis of the Polaroid is perpendicular (at 90°) to the plane of polarisation, all light is blocked/absorbed.

🧠 Exam Technique

  • Use elimination: The prompt states the intensity decreases to zero. Immediately rule out rows A and B because unpolarised light rotated behind a single filter never results in zero transmitted intensity.
  • Distinguish between C and D: Polaroid filters act by selectively absorbing light components parallel to their transmission axis (or passing them while absorbing perpendicular components), rather than "changing the plane of polarisation".

❌ Common Errors

  • Assuming unpolarised light can be completely blocked by a single rotating Polaroid filter.
  • Confusing the action of a Polaroid filter with optical activity (where certain substances rotate the plane of polarisation of light passing through them).

Topics

Physics · 3.3 Waves

Question and mark scheme from the AQA AS Level Physics examination, Paper 2, June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.